Create a minimum supported git version policy

We're constrained by supporting old git versions in https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1469 - it'd be nice to know what the minimum git version we're expected to support is, in these scenarios. "Support every version of git ever" is suboptimal.

To make good decisions here, we need to know which git versions come with the non-EOL versions of our supported platforms.

What are our supported platforms? The README says "Windows, OSX, Linux"... and mentions FreeBSD a bit further down.

Debian, Ubuntu, CentOS and RHEL come under "Linux" and these are the ones that will really hold us back, generally.

TODO: version matrix.