Return multiple errors and warnings in CI Lint API endpoint

What does this MR do?

• Removes method YamlProcessor.validation_message and uses YamlProcessor.new(...).execute instead
• Updates the method for the gitlab-ci.yml blob viwer, which was using YamlProcessor.validation_message and refactors it to say errors

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

#243732 (closed)