Support useJdkVersion flag and test JDK matrix via Gradle toolchains

[wmdietl]

Matches Checker Framework's convention for specifying the target JDK version using -PuseJdkVersion.

• Adds majorVersionToInt to parse -PuseJdkVersion, configuring the Java toolchain accordingly.
• Falls back to sourceCompatibility = 1.9 and targetCompatibility = 1.9 when -PuseJdkVersion is omitted.
• In CI (workflow.yml), passes -PuseJdkVersion=${{ matrix.jdk }} to ./gradlew build so matrix entries actually compile and test with their respective JDK versions under Gradle's Java toolchain while the Gradle daemon runs on JDK 21.
• Uses mkdir -p ~/.gradle in CI to avoid errors if the directory already exists.
• Replaces the for (int i = start; i < end; i++) loop in the test listener with (start..<end).each { i -> ... } to avoid the greclipse formatting bug that caused i <end.

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