Remove Unicorn support: main MR [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes unicorn gem, gemfile group, and license.

Previous MRs:

• Docs changes: !61923 (merged)
• Comments changes: !62010 (merged)
• Remove unused bin/pkgr_before_precompile.sh: !62079 (merged)
• Remove UnicornSampler: !62090 (merged)
• Update some individual specs, not directly related to Unicorn classes: !62107 (merged)
• Delete UnicornCheck service: !62204 (merged)
• Delete UnicornHttpServer mixin: !62219 (merged)
• Topology Schema description update: !62287 (merged)
• Update Unicorn-related binaries and init.d config: !62289 (merged)
• Remove unicorn-worker-killer and its relations: !62293 (merged)
• Remove Unicorn detection in pid provider: !62377 (merged)
• Remove Unicorn references from runtime logic: !62390 (merged)
• Remove gem: this MR

If some additional changes will be needed (e.g. more refactoring), they may be extracted into follow-ups under the parental epic.

GDK MRs:

• docs: gitlab-development-kit!2000 (merged)
• configs: gitlab-development-kit!2003 (merged)

GCK MR: gitlab-compose-kit!168 (merged)

Omnibus MR: omnibus-gitlab!5295 (merged)

Helm: WIP

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.

Availability and Testing

1. I grepped unicorn usages - nothing left outside docs, changelogs, and emojis having the same name
2. I performed an Omnibus update from 13.12 to the package generate by the CI on this branch, on Google Cloud. It went smooth:

• 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.

Security

N/A

Related to #330402 (closed)