Add baffling birthdays practice exercice

[Baboushka]

pull request

Add Baffling Birthdays exercise based on the problem specifications.

I set the exercise's difficulty to 6 and included the for-loops, lists, sets, randomness, datetime prerequisites.

The modifications passed:

• ./gradlew test
• ./gradlew check
• ./gradlew compileStarterTestJava
• bin/configlet lint (no errors for baffling-birthdays)

Notes:

• I added an extra test to verify birthdates follow the YYYY-MM-DD format (ref. 1st comment canonical-data)
• Randomness related tests use the average of multiple runs. I also added some tolerance to allow for small deviations (ref. 2nd+3rd comments canonical-data)

Closes #2931

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[kahgoh]

Thanks for contributing @Baboushka! Comments for this PR 👇🏻

[Baboushka]

Thank you for the review @kahgoh!
Your feedback is included in the latest commits: I updated to Gradle 9.2.1, removed comments and the extra test, and updated probability tests to match the C# approach.
The intermediary commit had indentation issues, that's corrected in the last commit.