[EE] Removes support for matching on `app` label

What does this MR do?

Removes support for using app label to match deployments and pods for:

• Deploy boards
• Kubernetes pod terminals
• Pod Logs

CE Backport MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29460

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/11209. Adding a warning about "why" their deployments may not be showing up on deploy boards will be in a later MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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