Add Managed Prometheus deprecation warning

What does this MR do?

Adds a deprecation notice for the prometheus Metrics dashboard and Alerts list.

Issue: #328774 (closed)

Screenshots (strongly suggested)

With managed prometheus

before	after

With manual prometheus (no change)

before/after

With no prometheus (no change)

before/after

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [n/a] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

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