GH-20161: [Website] Remove mentions of master branch from Apache Arrow website content

[kevingurney]

Overview

As part of apache/arrow#31142 and in response to the recent rename of the apache/arrow-site repository default branch to main, this pull request removes mentions of the "master" branch from the Apache Arrow website content.

Qualification

To qualify these changes, I:

1. Performed a local build and preview of the website using bundle exec rake and manually verified that all the links are still working.

Future Directions

1. There are still some remaining tasks listed in [Dev] Rename default branch to 'main' arrow#31142 related to the renaming of the default branch of the apache/arrow repo (and related repositories) to main.

Notes

Note: I left the mentions of "master" in the _release and _posts directory unmodified. As described in apache/arrow#20161 (comment), it's unclear whether it makes sense to change these uses since these are referring to a past state of the repository. If the community would prefer that we also removes mentions of "master" from these directories, I would be more than happy to do so.

Closes apache/arrow#20161.

[kevingurney]

Note: The CI failures in the Build step are unrelated to this change. This is a result of #325, which is being addressed separately.

[kevingurney]

@jorisvandenbossche, if you have the bandwidth, would you be willing to review this PR, as well?

If not, would you mind pointing me towards someone else in the community who might be interested in reviewing these changes?

Thank you!

[kou]

+1