Explicitly compile production code with java 8

This configures the main toolchain of the project to java 17, but it overrides the compiler for the compileJava task to use java 8, so compatibility with older java versions is guaranteed.

Gradle will now except dependencies that require newer java versions even if only test code is using newer versions, but this shouldn't be a problem since compilation should fail if incompatible classes are used in production code.

Author: pschichtel

conventions/src/main/kotlin/tel.schich.javacan.convention.base.gradle.kts

@@ -10,20 +10,20 @@ javaComponent.withVariantsFromConfiguration(configurations["testFixturesRuntimeE
 
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(8)
-    }
-
-    testing {
-        toolchain {
-            languageVersion = JavaLanguageVersion.of(25)
-        }
+        languageVersion = JavaLanguageVersion.of(17)
     }
 }
 
 tasks.test {
     useJUnitPlatform()
 }
 
+tasks.compileJava {
+    javaCompiler = javaToolchains.compilerFor {
+        languageVersion = JavaLanguageVersion.of(8)
+    }
+}
+
 tasks.withType<JavaCompile>().configureEach {
     options.compilerArgs.addAll(
         listOf(