FE: Add warning when creating scheduled scan execution policy for large groups

Why are we doing this work

Users might not be aware that creating scheduled scans for groups with a large number of projects can significantly impact performance. To prevent accidental performance degradation, we need to warn users about this in the Policy Editor of the Scan Execution Policy. This will allow them to make informed decisions about scheduling scans and avoid potential negative impacts on GitLab's performance.

Relevant links

• Epic
• Design

Non-functional requirements

• Documentation: a mention in the documentation about potential risk with enabling scheduled scan for large groups should be added
• Feature flag: no feature flag needed
• [-] Performance:
• Testing: verify that warning is displayed only on group level editor with large amount of projects,

Implementation plan

• Product Management Determine the threshold for "large" number of projects
• frontend Display a warning message in the Policy Editor when the group exceeds the threshold and allow users to bypass the warning and proceed with scheduling the scan.

Verification steps

1. Navigate to the Policy Editor for Scan Execution Policy for groups with many projects.
2. Try to create a scheduled scan and try to save the policy.
3. Verify that a warning message indicates the potential performance impact.
4. Verify that the user can bypass the warning and save the policy.
5. Now go to the Project policy editor and group with a low number of projects.
6. Try to create a scheduled scan and try to save the policy.
7. Verify that a warning message is not displayed.