Add flat-square badge style

What does this MR do?

Adds a new badge style called flat-square. It can be used with the style parameter in the badge image url. E.g.

https://example.gitlab.com/<namespace>/<project>/badges/<branch>/coverage.svg?style=flat-square

Screenshot

flat (default) and flat-square badge styles.

What are the relevant issue numbers?

#30120 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #30120 (closed)