Add language- prefix to CSS class of syntax highlighting blocks in markdown

What does this MR do?

Currently highlighted blocks in markdown have a CSS class with the name of the programming language. This CSS class may conflict with other classes (#220647 (closed)). This MR adds a prefix language- to avoid conflicts.

Screenshots (strongly suggested)

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