KAFKA-18400: Don't use YYYY when formatting/parsing dates in Java client

[kirktrue]

Correcting documented use of YYYY to be yyyy as used by SimpleDateFormat format.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[kirktrue]

Needs unit tests.

[AndrewJSchofield]

Thanks for the PR. I understand the distinction between year and week year, and it is certainly the case that the formatting and parsing should be using year (yyyy) and not week-year (YYYY). However, I feel the change of capitalisation of the format examples in the help messages is unhelpful to actual humans. If you want to be really pedantic about the use of SimpleDateFormat, the seconds and milliseconds should be ss.SSS, not SS.sss. However, in the help string, it's pretty clear what is meant.