ICU-22984 Generate old Java monkeys

[eggrobin]

Copied from the C++ in #3287 and hammered upon until it compiled and passed.

Checklist

• Required: Issue filed: ICU-22984
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[markusicu]

CI check failure:

Error:    RBBITestMonkey.TestLineMonkey:1694->RunMonkey:1459 » IllegalArgument Rule $NU × ($AL | $HL) found a break at a position which does not correspond to an index in the original string
Error:    RBBITestMonkey.TestRTLineMonkey:1763->RunMonkey:1459 » IllegalArgument Rule $NU × ($AL | $HL) found a break at a position which does not correspond to an index in the original string

[markusicu]

changes lgtm but the CI check still fails

[markusicu]

works in Java 11, 17, 21, but fails in Java 8; some Java regex behavior change? maybe skip these tests on Java 8??

[eggrobin]

Well, I ended up narrowing down the Java 8 inconsistency to something about surrogates, so I changed the code to not produce nor match them before Java 9.