Require Java 17+ for Maven runtime

* Decouple the Maven runtime JDK from the compile/test toolchain by
  introducing `hc.build.toolchain.version` and overriding the inherited
  `use-toolchains` profile to select the toolchain from that property
  instead of `maven.compiler.source`.

* Require Java 17 or newer to run Maven, but continue compiling and
  testing against Java 8, 11, 17, and 21 toolchains.

* Update CI to install JDK 17 for the Maven process and the matrix JDK
  as the toolchain, generate `toolchains.xml`, and pass
  `hc.build.toolchain.version` explicitly instead of disabling
  toolchains and running everything on the host JDK.

This does not drop support for Java 8 consumers: published artifacts
still target Java 8 and the build can still compile and run tests on
Java 8 through toolchains.