Bump minimum git version to v2.31.0

What does this MR do?

With git v2.31.0, several new features have been introduced which Gitaly wants to make use of:

- Passing git configuration via a new set of enviroment variables
  `GIT_CONFIG_COUNT`, `GIT_CONFIG_KEY_$n` and `GIT_CONFIG_VALUE_$n`.
  This allows us to configure credentials without leaking them via
  the command line.

- Atomic fetches, which allow us to make use of transactions when
  fetching many references via git-fetch(1).

- A performance optmization for fetches from a repository which has
  a huge amount of references.

Gitaly will thus bump the minimum required git version to v2.31.0. As a preparatory step, this commit updates GitLab to start requiring this version.

Downstream distributions of Gitaly (CNG, GDK, Omnibus) have been adapted to use Gitaly's make git target and thus always use the version specified by Gitaly, which is currently v2.31.1.

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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