Test deprecation of `text/plain` on all endpoints

What does this MR do?

Related to: !42229 (comment 435591364)

This tries to have a proper support for feature flag. This is kind of hacky, but maybe?

WDYT?

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