PIPE-4728 Add is default branch pipeline value

A new pipeline value that represents whether the branch the pipeline
was triggered on is the default, following the below criteria:

- `true` when it is the default branch.
- `false` when it isn’t the default branch.
- `false` when it is any other push (e.g. tag push).
- omitted when it is not a VCS-triggered pipeline.

This pipeline value is available to Cloud pipelines triggered after
31st July 2024 and Server 4.6+.

jekyll/_includes/snippets/pipeline-values.adoc

@@ -50,6 +50,13 @@
 | icon:check[]
 | icon:check[]
 
+| `pipeline.git.branch.is_default`
+| GitHub App, GitHub OAuth, Bitbucket, GitLab
+| Boolean
+| Whether the branch the pipeline was triggered on is the default branch.
+| icon:check[]
+| icon:check[] (>= v4.6)
+
 | `pipeline.git.revision`
 | GitHub App, GitHub OAuth, Bitbucket, GitLab
 | String