Bump minimum JRE and JDK to Java 21

[mgaffigan]

Support is extended to two prior LTS JRE's. Java 17 is no longer in that list. Java 21 brings new benefits, not least of which the use of UTF-8 as a default charset (#256).

[edthri]

So is the policy to support the latest LTS version (currently 25) and latest minus one (currently 21)?

[mgaffigan]

@edthri, correct. Not sure if that's written policy or just convention. See #18 (comment) for prior implementation of this policy.