The install instructions for CentOS still document how to build and
install git. This is tedious, as on each bump of the minimum required
git version for Gitaly, this documentation needs to be updated. As a
point in case, the currently documented minimum required version is
still documented as git 2.27, which hasn't been supported since release
v13.6.0 anymore.

To fix these issues, we've started migrating downstream distributions of
Gitaly to instead use the git version provided by Gitaly itself: it's
well tested, always up-to-date and thus overall a lot less of a hassle
to maintain. Source install users are already being told to install git
via Gitaly's `make git` target [1], so let's convert the CentOS install
documentation to do the same.

[1]: https://docs.gitlab.com/ee/install/installation.html#git