Add tooltips to user contribution graph key

What does this MR do?

Adds tooltips to the key values in the user contribution graph:

Color	Tooltip
#ededed	0 contributions
#acd5f2	1-9 contributions
#7fa8c9	10-19 contributions
#527ba0	20-29 contributions
#254e77	30+ contributions

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

nope

Why was this MR needed?

It was difficult to tell what each of the colored squares represented in the user contribution graph.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23751 (closed)