Add Pipeline Editor branch selector

This change enables users of the Pipeline Editor to select the branch to work on when editing the CI YAML file of their project.

Changelog: added

config/feature_flags/development/pipeline_editor_branch_switcher.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326189
 milestone: '13.11'
 type: development
 group: group::pipeline authoring
-default_enabled: false
+default_enabled: true

doc/ci/pipeline_editor/index.md

@@ -15,7 +15,7 @@ the `.gitlab-ci.yml` file in the root of your repository. To access the editor,
 
 From the pipeline editor page you can:
 
-- Select the branch to work from. [Introduced in GitLab 13.12](https://gitlab.com/gitlab-org/gitlab/-/issues/326189), disabled by default.
+- Select the branch to work from.
 - [Validate](#validate-ci-configuration) your configuration syntax while editing the file.
 - Do a deeper [lint](#lint-ci-configuration) of your configuration, that verifies it with any configuration
   added with the [`include`](../yaml/index.md#include) keyword.