Create Java 17 + Spring Framework 6 + Jakarta EE 9 platform

We now know that Spring Framework 6 will be out at the end of 2023, that it will have a minimum requirement of Java 17 (but will support the next two LTS releases as well, Java 23 and Java 29) and will switch from the Java EE APIs to Jakarta EE 9, thus requiring namespace changes.

This issue is a placeholder to wrap tasks related to exploring that environment, including experimenting with building our software stack under a new platform POM based on the same components.

This will kick off with experimental branches of our projects once a milestone of Spring Framework 6 is available, expected before the end of 2021.

The expectation is that we'd build a new platform and base a new IdP release on that, some time after Spring Framework 6 and the other dependencies are stable, possibly end 2022 to early 2023.