Fix Rubocop offences: ee/application_setting

What does this MR do and why?

Fix Rubocop with: bundle exec rubocop ee/app/models/ee/application_setting.rb -a
No other manual changes are done.

Extracted from !121565 (comment 1416191578)
The CI job was red until it was all fixed: failing job example
Moved the Rubocop fixes to a separate MR to reduce the diff of the original MR.

How to set up and validate locally

Reproduce:

• run bundle exec rubocop ee/app/models/ee/application_setting.rb on master vs this branch.
• Running on master reveals 81 offenses. No issues on this branch.

Validate:

• Primarily, rely on CI (no logic changes, only formatting)
• Open the Application Setting page locally, and make sure it works, no unexpected 500s.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.