Resolve "Do not backup external MR diffs during pre-install script"

What does this MR do?

Excludes External Diffs from the automatic backup taking during the preinst script

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Closes #8870

Test plan

1. On an existing installation that uses deb packages. Manually trigger the preinst script by running /var/lib/dpkg/info/gitlab-ee.preinst upgrade as the root user. You will see that External Diffs are included in the automatic backup
2. Upgrade to this package or apply the diff from this MR to /var/lib/dpkg/info/gitlab-ee.preinst and re-run /var/lib/dpkg/info/gitlab-ee.preinst upgrade. External Diffs should now be skipped in the automatic backup.