Add font color contrast to external label in admin area

What does this MR do?

Adds a darker font color to .label-default so that the labels have more contrast

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

I used the same font color as the .badge on that same page but it was not listed as a variable in variables.scss. I'm not sure if both references need to be added as a sass variable or whether they should be replaced with an existing color in the variables list.

In reference to Dmitriy's note about the colors, I changed the color to $btn-transparent-color and also took the liberty to change it on the navigation badges as well.

Why was this MR needed?

Improves existing UI

What are the relevant issue numbers?

Closes #20744 (closed)

Screenshots (if relevant)

Before:

After:

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