Use instance default branch name for wikis

Problem to solve

The default branch name master in Git appears to derive the name from it's predecessor BitKeeper which had master/slave naming. Aside from the history of the default branch naming in Git, the historic term master is problematic.

After adding an instance setting for default branch name for new repositories https://gitlab.com/gitlab-org/gitlab/-/issues/221013, new wikis should inherit from this setting when created.

Further details

Proposal

When a new wiki repository is created, the default branch (HEAD) should be inherited from the instance's default branch setting.

Testing

Please run package-and-qa in MR for regression testing.