loaddata: Fix random_datetime() edge case

[legoktm]

Status

Ready for review

Description of Changes

Instead of generating month and day separately, generate them at the same time by randomly selecting a day in the year, and then calculating the month and date based on that.

This should avoid randomly generating invalid dates like February 30th.

Fixes #7155.

Testing

• Visual review
• CI passes

Deployment

Any special considerations for deployment? n/a

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• These changes do not require documentation

[cfm]

Looks good after rebasing from develop after #7157. Ordinarily I'd want us to add a test case for a bug like this, but I concede it's probably not worth it for a development/test script.