Update Gitaly to 12.9.0-rc5

What does this MR do?

Updates Gitaly to 12.9.0-rc5. See the unreleased changelogs for more details.

This change also documents the new Praefect subcommand, reconcile, introduced in gitaly!1903 (merged).

Closes gitaly#2448 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done