The receive.autogc configuration apparently needs to be in its own
file, not in the general gerrit.conf. Move it to the correct
location.
https://review.opendev.org/Documentation/config-gerrit.html#jgit-receive
While we're here, correct the filename on the gerrit.config and
secure.config templates to make it clear they're jinja2 files, and
add a file mode to the replication.config where it was missing.
Change-Id: I9243bccac103c51ee435725aae482731642a37cc