Use core.fsync as opposed to core.fsyncObjectFiles

We are still using the deprecated core.fsyncObjectFiles in /var/lib/gitlab/.gitconfig (which is generated from this file: https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/files/gitlab-cookbooks/gitlab/attributes/default.rb#L30)

Running git commands on the server (to fix issues in the repositories) will return the following warning:

warning: core.fsyncObjectFiles is deprecated; use core.fsync instead