Strip pre-java-9 version check in Metrics as Velocity targets 21+

[WouterGritter]

As Velocity builds to target java 21, there's really no need to support legacy version parsing.

[kennytv]

Runtime.version() contains the feature version and extras

[WouterGritter]

Runtime.version() contains the feature version and extras

Runtime.version().toString() will produce an output like 21.0.1+12-LTS, not the 21.0.1 that System.getProperty("java.version") provides.

I'd propose the following refactor:

metrics.addCustomChart(new DrilldownPie("java_version", () -> {
  Runtime.Version version = Runtime.version();

  return Map.of(
      "Java " + version.feature(),
      Map.of(System.getProperty("java.version"), 1));
}));

but it seems a bit odd to mix the old "api" (system property) and the new Runtime.Version API here. Thoughts?

Edit: I think it goes without saying that we shouldn't mess up the output to divide bStats metrics after this PR.

Edit 2: See latest commit, it's pretty easy to reconstruct the java.version system property exactly given a Runtime.Version

[kennytv]

Mixing is fine I'd say

.

[WouterGritter]

Mixing is fine I'd say

Better than the javaVersion helper method? I'm leaning more towards that approach (the current state of the PR)