Project push rules: Ensure that `regexp_uses_re2` column from `push_rules` is always `true`

Problem

Field regexp_uses_re2 in push_rules was created to introduce a RE2 regex syntax for push rules a long time ago. See commit: 206166e2.

All push rules should have regexp_uses_re2 set to true by now. But we need to verify that and migrate records with regexp_uses_re2 = false.

Proposal

1. Verify the status of regexp_uses_re2 on GitLab.com. Are there any regexp_uses_re2=false records in the database?
2. Create a migration plan for self-managed instances.