**Problem**
Running sbt 2.x with JDK 8 produces a confusing "server was not
detected" error because the JDK version check only required JDK 8+
and only ran in the non-native-client path.

**Solution**
Move java_version detection before the native client decision and add
checkJava17ForSbt2 that requires JDK 17+ when sbt major version >= 2.

Fixes sbt#8813

* [2.x] fix: Fail early when sbt 2.x is run with JDK < 17 (sbtw)

Move JDK version check before native client decision in sbtw and
require JDK 17+ when build.properties declares sbt 2.x.

* [2.x] fix: Fail early when sbt 2.x is run with JDK < 17 (sbt.bat)

Move checkjava before native client decision in sbt.bat and require
JDK 17+ when build.properties declares sbt 2.x.

* [2.x] test: Add minimumJdkVersion helper and unit tests for sbtw

Extract JDK version check logic into Runner.minimumJdkVersion for
testability. Add RunnerSpec with tests for sbt 1.x, 2.x, and 3.x
version detection.

* [2.x] test: Bump fake java to JDK 17 for integration tests

The fake java script used by launcher integration tests reported
JDK 8. Since sbt 2.x now requires JDK 17+, the citest2 (sbt 2.x)
integration tests would fail with the new JDK version check.

* Simulate JDK 9+ rt.jar handling in fake java script

Instead of silently ignoring --rt-ext-dir (which causes sbt.bat
to mkdir on an empty string), properly simulate JDK 9+ behavior
by creating a temp directory with java9-rt-ext- prefix and a
dummy rt.jar inside it.