Check for Knative installation on enabled clusters

What does this MR do?

Fixes #38288 (closed)

For manual QA, check if all 3 are true:

• on a clean project without making any deployments, with a group cluster associated with Knative installed on it (no other clusters associated), we should go to Operations > Serverless on the project page and check that we see: "No functions available" instead of "Install Knative"
• on a clean project without making any deployments, with an instance cluster associated with Knative installed on it (no other clusters associated), we should go to Operations > Serverless on the project page and check that we see: "No functions available" instead of "Install Knative"
• on a clean project without making any deployments, with a project cluster associated with Knative installed on it (no other clusters associated), we should go to Operations > Serverless on the project page and check that we see: "No functions available" instead of "Install Knative"

In other words, if any cluster on any level associated to a project has Knative installed on it, and before making any deployments, when we go to Operations > Serverless on the project page, we should "No functions available" and not "Install Knative".

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

Closes #38288 (closed)