Split monitor visiblity setting from operations

What does this MR do and why?

This separates the visibility settings of Monitor from the operations setting it is currently controlled by.

This is behind a Feature flag, split_operations_visibility_permissions, and is part of a wider Epic: Adapt visibility settings to new structure afte... (&8004), where we are splitting the Operations toggle into 3 others.

When split_operations_visibility_permissions is enabled:

• The Visibility settings for the Project will now show a Monitor option, and will no longer show the Operations option.
• The Monitor item in the sidebar will be controlled by the Monitor visibility setting.
• Disabling the monitor visibility setting will also disable the metrics_dashboard, sentry_issue, and alert_management_alertfeatures.

Refs [Feature flag] Rollout of split_operations_visi... (#364240 - closed)

Screenshots or screen recordings

FF ON	FF OFF
		setting on
		setting off

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

1. In Rails console: Feature.enable(:split_operations_visibility_permissions)

2. Toggle Monitor via Settings > Visibility, project features, permissions

3. Watch Monitor sidebar to appear/disappear

4. In Rails console: Feature.remove(:split_operations_visibility_permissions)

5. Toggle Operations via Settings > Visibility, project features, permissions

6. Watch Monitor sidebar to appear/disappear

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #361585 (closed)