Compatibility with JDK 15

[sbrannen]

Along the lines of #24453, this issue aims to track JDK 15 compatibility.

For the time being (i.e., during the 5.2.x time frame), we'll simply set up a CI build compiling and executing tests against JDK 15 early access builds.

[sbrannen]

We now have a JDK 15 CI build plan in place: https://build.spring.io/browse/SPR-JDK15

[jhoeller]

Chances are that we'll officially support JDK 15 in the 5.2.x line still, giving it a JDK 8-15 range overall, expecting demand for early adoption there. The 5.3.x line is then expected to launch with a JDK 8-15 range, embracing JDK 16 and 17 in the course of its lifetime and therefore ending up with JDK 8-17.

We'll reschedule this ticket accordingly once a corresponding 5.2.x milestone is available on GitHub.