Switch CI pipeline to a JDK17 baseline

[bclozel]

As announced on the spring.io blog, Spring Framework 6.0 will require JDK17 as a baseline.

As a result, the default build pipeline should use JDK17 (with future variants for JDK18+) and the published artifacts should use the Java 17 language level.

This task should refactor the current CI pipeline to:

• only ship 17+ JDKs in the CI container image
• consistently use Java 17 language level for compiling main and test sources
• for now, Kotlin doesn't officially support Java 17, so we'll move to JDK 11 for now