Fix contributions calendar month label truncation

What does this MR do?

Expands the width of the contributions calendar by 3px when the last column is a new month to prevent the month label from being truncated

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

Shouldn't be

Why was this MR needed?

Fixes an existing UI issue

What are the relevant issue numbers?

Closes #20844 (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