yamllint: Enable and configure `braces` rule

What does this MR do and why?

Enforce braces to be used with a single space inside. Empty braces don't have a space:

  good: { this_is: ok }
  empty: {}  # Also OK

  bad: {this_is: too_little}
  also_bad: {  this_is: too_much  }
  empty: { }  # Also bad

Contributes to #385693.

How to set up and validate locally

scripts/lint-yaml.sh . $(find . -name "*.yml.*" -or -name "*.yaml.*" | grep -vE '\.(erb|gz|bz2|swp|swo)$')

MR acceptance checklist

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