git: Make the Gitaly Git version override version-agnostic

What does this MR do?

With 89950647 (Set GIT_REPO_URL, GIT_VERSION_2_37_1, GIT_VERSION_2_38, 2022-11-11), we have introduced the logic to allow overriding the bundled Git versions that Gitaly is using. Back then we had been hard coding the specific versions that could be overridden. But as these bundled Git versions are under the control of Gitaly and change over time those are bound to become out-of-date.

In fact, they already are out-of-date as Gitaly only distributes the Git v2.38 release train. Furthermore, Gitaly will soon add a second release train with Git v2.39, which would require another new variable override.

Refactor the logic to be version-agnostic so that we just forward all environment variables that have the GITALY_GIT_VERSION_ prefix to the build environment.

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened