Instruct users to build git via Gitaly

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.

Part of gitaly#3512 (closed)