Fixing single string values in the 'include:' keyword of gitlab-ci.yml validations

What does this MR do?

This fix is a followup to !24098 (merged) which introduced a validation of the include: keyword of a gitlab-ci configuration file when triggered from /ci/lint API calls.

However, there was a test case missing: the case of a single string as value. I have added a test case for that which shows that the code was not validating it correctly.

This commit fixes that to allow all include: valid inputs.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

A unit test showing the missing case has been added.

Security

N/A