Use instance default branch name for wikis

What does this MR do?

In this MR, we change the default branch for the wiki repository after creating it. For that, we're changing the symbolic ref inside the HEAD file, populating it with the instance default branch.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #221159 (closed)