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

Gitaly is upgrading its minimum required Git version to v2.29.0. This
commit updates the CentOS instructions to require this new version.

Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>

The default is now Git 2.27, so let's update `install/centos/README.md`
to reflect that.

Part of: gitaly#2829

The default is now Git 2.26, so let's update `install/centos/README.md`
to reflect that.

Part of: gitaly#2497

Gitaly now supports new minimum version of Git because it depends on the new features from it, please see gitaly!1653

Part of: gitaly#2170

Git solved a security issue for partial clone in 2.22, a feature GitLab
doesn't support yet, but we hope to in the future.

Part of: gitaly#1715

Update info about Caddy deploy.

Add gitlab-gitaly.service to systemd instructions

FIX: incorrect order of defined methods in import/bitbucket script

Fix typo