Bring back Gitlab::UntrustedRegexp.with_fallback

What does this MR do?

Fixes #10864 (closed)

Bring back Gitlab::UntrustedRegexp.with_fallback as it's still being called in PushRule model.

Also still present in 11-9-stable-ee branch which was release as 11.9.4. This commit in 11-9-stable-ee branch (https://gitlab.com/gitlab-org/gitlab-ee/commit/c5ae609534cf5cf75a13b066f2569ae4fb88761d) doesn't remove the with_fallback method.

That commit includes other changes though which is unrelated so I just copy pasted back with_fallback based on https://gitlab.com/gitlab-org/gitlab-ee/blob/11-9-stable-ee/lib/gitlab/untrusted_regexp.rb#L50-61.

What are the relevant issue numbers?

#10864 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer