Add context to full code quality report

What does this MR do?

For #231341 (closed)

Adds the sentence "This report is all Code Quality issues in the source branch." as a sub-heading on the code quality report for a pipeline, in order to clarify that the report contains all of the code quality issues that exist in the branch/commit and not just issues that were introduced or fixed (which is what the code quality MR widget does).

Screenshots

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done