yamllint: Enable all rules

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Problem

In #359968 (closed) we've started linting YAML in CI.

In order to make the first step easier we've disabled some yamllint rules but added a note to enable them at some point. This point is now.

Proposed solution

Enable all rules in yamllint.

Process

1. Pick a rule from the the list below
   • Tick the checkbox
2. Enable or configure this rule
   • Remove this rule from .yamllint to enable it with default values
   • Configure a different value
   • See https://yamllint.readthedocs.io/en/stable/rules.html
3. Check offenses
   • Run scripts/lint-yaml.sh . $(find . -name "*.yml.example")
4. Fix the offenses
   • ⚠️ Note that some YAML files a linted with a different config. See https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/ci/yaml.gitlab-ci.yml
   • These rules are marked with ⚠️ because they will produce a lot of offense and are harder to fix
5. Create MR
   • Link that MR next to the item being fixed below
6. 🎉

The list

• braces: disable - !106945 (merged)
  • 🆕 braces: { min-spaces-inside: 1, max-spaces-inside: 1, min-spaces-inside-empty: 0, max-spaces-inside-empty: 0 }
• colons: disable
• comments-indentation: disable
• comments: disable
• document-start: disable ⚠️
• empty-lines: disable
• indentation: disable ⚠️
• key-duplicates: disable
• line-length: disable ⚠️
• new-line-at-end-of-file: disable
• trailing-spaces: disable
• truthy: disable

Follow-ups

The item marked with ⚠️ might need more attention and could be resolved in follow-up issues.