Change default attachment size limit to 100MB

Problem to Solve

As part of #20061 (closed) the upload attachment limit on GitLab.com was increased to 100MB. As part of our product principle for configuration this limit should also be reflected in self-managed installs as the new default.

Proposal

The default upload limit should be changed to 100MB in self-managed.

Additional Details

We should consider only adjusting the limit for either net-new installations of GitLab or for installations that have not changed from the old 10MB limit. This will prevent any unexpected changes where an organization may have lifted this above 10MB and it's changed again for them.