Resolve "Set the size of instance system message text 12px"

What does this MR do?

• Updates system message header/footer text to use $gl-font-size-small (12px)
• Remove duplicative declaration of $system-header-height, to avoid any overriding.

Screenshots

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/59131

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• Security reports checked/validated by reviewer

Closes #59131 (closed)