Switch JAVA_HOME to 21 for JRuby (#721)

* JRuby 10 requires Java 21. Since the previous default was 17 and all JRuby releases should work fine on 21, we do this for all JRuby installs.
* Implements #718

common.js

@@ -6,6 +6,7 @@ const stream = require('stream')
 const crypto = require('crypto')
 const core = require('@actions/core')
 const tc = require('@actions/tool-cache')
+const exec = require('@actions/exec')
 const { performance } = require('perf_hooks')
 const linuxOSInfo = require('linux-os-info')
 
@@ -403,3 +404,33 @@ export function setupPath(newPathEntries) {
   core.addPath(newPath.join(path.delimiter))
   return msys2Type
 }
+
+export async function setupJavaHome() {
+  await measure("Modifying JAVA_HOME for JRuby", async () => {
+    console.log("attempting to run with existing JAVA_HOME")
+
+    let ret = await exec.exec('ruby', ['--version'])
+
+    if (ret === 0) {
+      console.log("JRuby successfully starts, using existing JAVA_HOME")
+    } else {
+      console.log("JRuby failed to start, try Java 21 envs")
+
+      let arch = os.arch()
+      if (arch === "x64" || os.platform() !== "darwin") {
+        arch = "X64"
+      }
+
+      let newHomeVar = `JAVA_HOME_21_${arch}`
+      let newHome = process.env[newHomeVar]
+
+      if (newHome === "undefined") {
+        throw new Error(`JAVA_HOME is not Java 21+ needed for JRuby and \$${newHomeVar} is not defined`)
+      }
+
+      console.log(`Setting JAVA_HOME to ${newHomeVar} path ${newHome}`)
+
+      core.exportVariable("JAVA_HOME", newHome)
+    }
+  })
+}