Allow creation of group-level custom-roles on self-managed instances

What does this MR do and why?

• In 16.8, creating group-level custom role creating on a self-managed instance results in an error.

In this MR:

• Introduce beta feature-flag, restrict_member_roles, which is disabled by default.
• When it's off, admins can create group-level and instance-level custom roles when on a self-managed instance.
• When it's on, admins can only create instance-level custom roles on a self-managed instance.
• When on SaaS mode, only group-level roles can be created.
• Merged to Gitlab.com - !142637 (merged) + !141798 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
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