Remove GitLab-managed applications UI

What does this MR do?

A conservative removal of the GitLab-managed apps functionality, achieved by

1. hiding the now deprecated applications tab from cluster show page, and
2. hiding the external ingress IP from the cluster details tab
3. removing the one-click section from the documentation

To facilitate reviews, I removed just enough deprecated tests to get the pipelines passing.

NOTE: Alert creation is being disabled in !62974 (merged)

There is a lot of code still to be removed in a follow up, and the unused tables will be dropped in the earliest %14.1 as they are required for a pair of data migrations that were landed in the early phases of %14.0.

Majority of the documentation was updated in !63257 (merged).

Screenshots (strongly suggested)

Observe that

1. there is no Applications tab anymore
2. there is no ingress .nip.io domain suggestion
3. there is a removal warning banner

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.