Handle schedule linked to escalation rule removal - simple case

What does this MR do?

When oncall-schedule that escalation rule is linked to is deleted, the rule in the escalation policy is deleted as well. So we can have a situation where the policy has no rules. For the first pass we'll simply add a warning message to the delete schedule modal + when policy has no rules - display a red warning instead of the rules.

A more sophisticated approach is still discussed.

This fix is essential for the feature flag removal

To check: Enable escalation_policies_mvc feature flag

The minimal license required is GitLab Premium

Navigate to your project=>Monitor=>Escalation Policies.

Add escalation policy with some schedule

⚠ If schedule dropdown in add policy modal is disabled - navigate to your project=>Monitor=>Oncall-schedules to create a schedule

Delete a schedule that was linked to the policy

Observe that policy without rules has a warning

Screenshots or Screencasts (strongly suggested)

Schedule confirmation message

Empty rules warning

Does this MR meet the acceptance criteria?

Conformity

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #334739