Added showHeader prop to monitor dashboard

What does this MR do?

The monitoring dashboard relies on the presence of environmentsEndpoint variable to show/hide the header (with buttons).

This is not a very clean way and also, I have another MR !23659 (merged) that explicitly removes environmentsEndpoint occurrences which will break the cluster health page.

This MR introduces showHeader prop for the dashboard component to cleanly handle this situation.

Note Because this a developer-only change there is a changelog file.

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

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:

• 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