Remove vendor prefixes for linear-gradient CSS

What does this MR do?

Removes the vendor prefixes for linear-gradient and the linear-gradient mixin since they are no longer needed with our latest and greatest browser (as per annabeldunstone)

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

None

Why was this MR needed?

Reduces unnecessary CSS

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• Closes #21558 (closed)