Replace contributions calendar timezone payload with dates

What does this MR do?

Fixes a regression bug introduced in !5784 (merged) whereby the calendar squares would not render due to a timezone mismatch between the server timezone and the browser (user) timezone. I am now returning the dates instead of the timezone from the backend. There's a good chance we will still have a calendar off by one error (which is an existing issue #1943 (closed)) but passing dates should make sure that the calendar activity squares render correctly across timezones.

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

Double check that this displays the correct calendar squares when the GitLab instance is hosted in a different timezone than the user viewing it

Why was this MR needed?

Fixes regression issue 😞

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21921 (closed)