Reference compatibility mode mixin with its full name in the release notes

[romaricpascal]

What

Reference the govuk-compatibility-mode mixin with its full name, rather than just compatibility-mode (within '"Compatibility mode" features are no longer supported')

Why

This will help users search their codebase for the use of that mixin without risk of collision with another piece of code that's named < SOMETHING >compatibility-mode< SOMETHING>.

Who needs to work on this

Developers, Tech writers

Who needs to review this

Developers, Tech writers

Done when

• [x] The mixin is referenced by its correct name
• [ ] We've checked if other naming mishapps are in the release notes

[36degrees]

Added a suggestion in the doc that resolves this

[claireashworth]

Accepted suggestion in doc that resolves this

[romaricpascal]

Updated in the draft changelog (internal)