Backport from EE: Fix color of text in resolution checkbox

What does this MR do?

Backports the change from EE branch, to add consistency to note_form.scss in both EE/CE.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7944

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides