Do not backup external MR diffs during pre-install script

Problem

From gitlab!154914 (comment 2272835042):

Does external_diffs need to be added to automatic omnibus db backup command - https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/4e189c8ee0df821c4a8a3b0955dcc86158558e6b/config/templates/package-scripts/preinst.erb#L97?

More details

• External MR diffs began to be backed up as well during pre-install since 17.1.

Proposal

external_diffs should be skipped during the GitLab pre-install script.

The intent is clear: notify "Automatically backing up only the GitLab SQL database (excluding everything else!)"