Remove Unicorn references from `runtime.rb` and config examples

What does this MR do?

This MR is split from big Unicorn removal MR: !61888 (merged) .
MR description includes the full list of the merge requests being split.
We plan to drop everything Unicorn-related in GL14.0 .

This MR address:

• Unicorn references in runtime.rb
• Unicorn config .example files
• Related tests
• Comment update (related to rbtrace)

After this MR being merged (alongside with !62293 (merged) and !62377 (merged)), the next step should be the removal of the gem, license references to the gem, and :unicorn Gemfile group references - should be the final one in the GitLab repo.

Omnibus MR: omnibus-gitlab!5295 (merged)

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 checked locally in GDK
2. I performed an update from 13.12 to the .deb produced by the CI in this branch, on GCK (Omnibus installation):

(it's funny that it marked that as 13.13)

I haven't encountered any issues.

• 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)