Enable new RPC call to retrieve wiki files [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

In this MR we remove the feature flag gitaly_find_file enabling the functionality to replace the existing wiki#find_file RPC call to the Gitaly WikiService with a different RPC.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #324306 (closed)