Backmigrate to Java 17

[jottinger]

GraphCompose right now uses Java 21 features at the source level and at the library level. It would be useful to backport to Java 17; it makes the source a little uglier, but shouldn't affect performance, and widens the audience to the set of Spring Boot 4 users as well as a host of others.

[DemchaAV]

Closed via v1.6.1 — Java 17 baseline migration shipped. Thanks @jottinger for the foundational work in #11 and #12.