plugin: Try to get "jdk 1.8" from toolchains.xml, bump Maven to >= 3.3.9

[kohlschuetter]

Previously, we were OK getting any JDK version from the build context.

This breaks the Maven plugin when executing with a too new Java version (e.g., 20).

Filter through the toolchains available in toolchains.xml, and use the first working one that is marked "jdk 1.8".

This change requires updating the Maven Plugin API requirement.