EE: Allow to use untrusted ruby syntax

What does this MR do?

Brings an optional administratively controlled backward compatibility to use unsafe Ruby Regexp with only: and refs:.

This also brings back the compatibility for: // =~ 'aa' which become broken due to PATTERN.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/59703.

The CE version: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26905

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Style guides

Performance and testing

• Tests added for this feature/bug

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team