Update gitaly version and FindChangedPaths

What does this MR do and why?

This merge request updates Gitaly gem versions to the latest version 18.6.0.pre.rc1.

The new version adds the ability to request commit_id from FindChangedPaths. This will be used by groupsecret detection to improve the performance of Secret Push Protection as it will no longer require calling GetTreeEntries() RPC per each commit scanned to determine the commit sha/file path of a secret finding.

We additionally update GITALY_SERVER_VERSION to the commit with this new functionality.

We also update FindChangedPaths() RPC on the Rails side to use this new commit_id field.

References

• This part of the necessary groundwork to remove GetTreeEntries() from SPP.

MR acceptance checklist

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