Test JsonNullable as a record component
Merged
SE
25d ago+89+89+01234567890123456789-0-0-0123456789
JsonNullable already works as a record component on Jackson 2 and 3, but nothing tested it. This adds a RecordTest for both and a README paragraph. Test sources now compile at release 17; main stays at 8.
Summary by cubic
Adds tests verifying JsonNullable works as a record component on Jackson 2 and 3, and documents this behavior in the README. Test sources now compile at Java 17; main sources remain on Java 8.