Backport shared example fix in 19-0-stable-ee

What does this MR do and why?

This fixes a spec failure first identified in #600471 (closed).

The 'when namespace enforces granular tokens' before block was introduced on this branch by !229817 (merged) (which also added the granular_personal_access_tokens_enforcement_saas flag and the enforcement logic). Its original setup called root_ancestor.create_namespace_settings!, which INSERTs a namespace_settings row and fails with a PG::ForeignKeyViolation when the referenced namespace is not present in the current transaction (e.g. a let_it_be group boundary object). Simply removing that insert but keeping root_ancestor.namespace_settings then fails with NoMethodError: undefined method 'update!' for nil, because the association is nil on the freshly-loaded root_ancestor.

Align the setup with master: gate on root_ancestor&.group_namespace? and update the existing row via NamespaceSetting.find_by!(namespace_id: root_ancestor.id).

Backports the shared-example changes from !235703 (merged) (the skip guard and removal of create_namespace_settings!) and !238989 (merged) (the switch to NamespaceSetting.find_by!) on master.

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).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

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

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.