Resolve link hover+focus consistency issues

What does this MR do?

Adds a mixin for the hover+focus state of links, which sets the color and underline.

Applies this mixin on all the links we need it on.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

This affects 90% of pages so this can only be reviewed by checking out. We have plenty of time to review this, conflicts shouldn't be too hard to keep up with so please take your time. :)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24129 (moved)

Closes #24125 (moved)

Closes #24130 (moved)