build(jvm): define java toolchain to be 21

this should force gradle to run all tasks under java 21, so we stop getting partner support requests when they try to build the repo with java17 instead of 21.

Author: rli

build.gradle.kts

@@ -71,6 +71,14 @@ tasks.coverageReport {
 }
 
 allprojects {
+    plugins.withType<JavaPlugin>().configureEach {
+        java {
+            toolchain {
+                languageVersion = JavaLanguageVersion.of(21)
+            }
+        }
+    }
+
     tasks.configureEach {
         if (this is JavaForkOptions) {
             jvmArgs("-XX:ErrorFile=${rootProject.file("build/reports").absolutePath}/hs_err_pid%p.log")