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.