docs(contributing): Replace usage of "master" with "main" or "leader"

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 # Mixxx Contribution Guidelines
 
-Thank you for contributing to [Mixxx](https://mixxx.org/)! Your work helps DJs all over the world! We are global, all volunteer team that works by consensus and we are excited to have you join us. This document specifies technical aspects of our workflow. For social aspects please refer to [CODE_OF_CONDUCT.md](https://github.com/mixxxdj/mixxx/blob/master/CODE_OF_CONDUCT.md) in this repository. We encourage you to introduce yourself on our [Zulip chat](https://mixxx.zulipchat.com/) before starting to contribute code to Mixxx.
+Thank you for contributing to [Mixxx](https://mixxx.org/)! Your work helps DJs all over the world! We are global, all volunteer team that works by consensus and we are excited to have you join us. This document specifies technical aspects of our workflow. For social aspects please refer to [CODE_OF_CONDUCT.md](https://github.com/mixxxdj/mixxx/blob/main/CODE_OF_CONDUCT.md) in this repository. We encourage you to introduce yourself on our [Zulip chat](https://mixxx.zulipchat.com/) before starting to contribute code to Mixxx.
 
 Table of Contents
 
@@ -44,7 +44,7 @@ All of these are automatically built and deployed by [GitHub Actions](https://gi
 
   Neither is this:
 
-      fix a bug with quantize while the deck is playing and master sync is enabled and an effect unit is on the deck while the user is turning an EQ knob
+      fix a bug with quantize while the deck is playing and sync leader is enabled and an effect unit is on the deck while the user is turning an EQ knob
 
   Refer to [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/) for more details.
 
@@ -62,8 +62,8 @@ All of these are automatically built and deployed by [GitHub Actions](https://gi
 
 * Generally, prefer merging over rebasing. Do not rebase unless you have discussed that with whoever is reviewing the pull request. When you rebase a branch with an open pull request, it is no longer possible to distinguish your latest changes from already reviewed parts, resulting in unnecessary extra work for the reviewer. Comments made directly to a single commit will be lost. Rebased commits are likely not tested and there is a risk that building fails in a later `git bisect` run. If you want to correct minor mistakes with a rebase within a few minutes of pushing commits, that is okay as long as no one has started reviewing those commits yet. A `git commit --amend` is possible at any time as long the commit has the limited scope of one topic.
 * If you are helping with someone else's pull request that is not yet merged, open a pull request targeted at their fork. Leave a comment on the upstream pull request (which targets mixxxdj/mixxx) with a link to your pull request so other Mixxx contributors are aware of your changes.
-* Low risk bug fixes should be targeted at the stable branch (currently 2.2). However, bug fixes for the stable branches must have a direct impact on users. If you spot a minor bug reading the code or only want to clean up the code, target that at the master or beta branch.
-* Controller mappings should be targeted at the stable branch unless they use features that are new in the beta or master branch.
+* Low risk bug fixes should be targeted at the stable branch (e.g., `2.3`). However, bug fixes for the stable branches must have a direct impact on users. If you spot a minor bug reading the code or only want to clean up the code, target that at the `main` or beta branch.
+* Controller mappings should be targeted at the stable branch unless they use features that are new in the beta or `main` branch.
 * If you are making changes to the GUI with a pull request, please post before and after screenshots of the changes.
 * Please help review other people's pull requests. When others review your pull requests, please return the favor. The continued progress of Mixxx depends on all of us working together. Even if you are not familiar with the area of the code being changed in a pull request, you can be helpful by building the branch, verifying that it works as described, and commenting with feedback about the user experience design.
 * If you demonstrate good coding skills, help review pull requests, contribute major features, and show a commitment to Mixxx over time, we may invite you to the core team.
@@ -79,7 +79,7 @@ Mixxx core team members are contributors who have write access to the [upstream
 * You may merge someone else's pull request as the only reviewer if no other contributors have expressed concerns about the changes or said they want to review the code. Please do not merge pull requests immediately; allow at least a day or two for others to comment. Remember we are all volunteers and cannot respond to everything immediately.
 * If there is disagreement about changes in a pull request, do not merge it until a consensus has been reached.
 * Check CI to ensure builds work and tests pass before merging. If CI timed out, either manually restart it or build the branch and run the tests locally before merging.
-* When you merge a pull request to a stable branch, merge the stable branch to the beta branch afterwards. If you merge a pull request to a beta branch, merge the beta branch to master afterwards. When backporting, cherry-pick or rebase rather than merge.
+* When you merge a pull request to a stable branch, merge the stable branch to the beta branch afterwards. If you merge a pull request to a beta branch, merge the beta branch to `main` afterwards. When backporting, cherry-pick or rebase rather than merge.
 * Merge PRs using a merge , to keep the original commits valid. Keep the default commit message "Merge pull request ..." with the reference to the pull request. In case where the PR contains broken (non-building) commits, back-and-forth commits or commits without a meaningful commit message that are not worth keeping, ask the author to squash the commits before merge or use squash-and-merge if the author is not fluent in git.
 * Default to open; only post in the private Zulip stream for discussions that have a reason to be private. Most of the time, post to a public Zulip stream so anyone can participate in the discussion.
 * When Mixxx participates in Google Summer of Code, you may volunteer as a mentor if you like.