Draft: Set gitaly configuration to use transactions enabled by default

What does this MR do and why?

With gitaly!6496 (merged) gitaly will be able to use transaction management. As this is will be the prefered mechanism for running gitaly let's enable it for testing gitaly#5664 and in turn to validate production.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

• Check the pipeline jobs logs for the gitaly service and ensure a log line exists Transactions enabled. Transactions are an experimental feature. The feature is not production ready yet and might lead to various issues including data loss. See https://gitlab.com/gitlab-org/gitaly/-/blob/348c0477123886ba100b17e1858340d5453adca0/internal/cli/gitaly/serve.go#L369

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.