Reduce height of instance system message

Problem to solve

In Admin - Appearance - Header/Footer message. Height of message should be changed to 16px to fit customer's style guide.

Target audience

Related to this MR done in support of contractual obligations in US Public Sector organization.

All users will see the banner.

Further details

Feature was built as part of public sector work program and they request the styling to be changed.

Proposal

Change pixel size of instance header/footer message to 16px

Permissions and Security

N/A

Documentation

No changes necessary to https://docs.gitlab.com/ee/customization/system_header_and_footer_messages.html

What does success look like, and how can we measure that?

Having both header and footer messages be 16px

Links / references