Switch build-tools to latest target version (#34746)

- we already require Java 11 to build, yet we target the minimum
supported version in build-tools ( currently 8 )
- this is because we have some checks that are executed in a new JVM
which could be running the minimum version.
- For everything else it would be nice to be able to use new features,
like the new process API.

With this change, we selectively compile the few classes that need an
older target version and move everything over to Java 10.
Unfortunately the current Gradle version does not support 11 as a target
version yet.

buildSrc/build.gradle

@@ -31,22 +31,12 @@ if (GradleVersion.current() < GradleVersion.version(minimumGradleVersion)) {
     throw new GradleException("Gradle ${minimumGradleVersion}+ is required to build elasticsearch")
 }
 
-if (JavaVersion.current() < JavaVersion.VERSION_1_8) {
-  throw new GradleException('Java 1.8 is required to build elasticsearch gradle tools')
-}
-
 if (project == rootProject) {
   // change the build dir used during build init, so that doing a clean
   // won't wipe out the buildscript jar
   buildDir = 'build-bootstrap'
 }
 
-// Make sure :buildSrc: doesn't generate classes incompatible with RUNTIME_JAVA_HOME
-// We can't use BuildPlugin here, so read from file
-String minimumRuntimeVersion = file('src/main/resources/minimumRuntimeVersion').text.trim()
-targetCompatibility = minimumRuntimeVersion
-sourceCompatibility = minimumRuntimeVersion
-
 /*****************************************************************************
  *         Propagating version.properties to the rest of the build           *
  *****************************************************************************/
@@ -82,6 +72,45 @@ processResources {
   from tempPropertiesFile
 }
 
+
+if (JavaVersion.current() < JavaVersion.VERSION_1_10) {
+  throw new GradleException('At least Java 10 is required to build elasticsearch gradle tools')
+}
+
+/*****************************************************************************
+ *         Java version                                                      *
+ *****************************************************************************/
+
+// Gradle 4.10 does not support setting this to 11 yet
+targetCompatibility = "10"
+sourceCompatibility = "10"
+
+// We have a few classes that need to be compiled for older java versions because these are used to run checks against
+// those
+sourceSets {
+  minimumRuntime {
+    // We only want Java here, but the Groovy doesn't configure javadoc correctly if we don't define this as groovy
+    groovy {
+      srcDirs = ['src/main/minimumRuntime']
+    }
+  }
+}
+compileMinimumRuntimeGroovy {
+  // We can't use BuildPlugin here, so read from file
+  String minimumRuntimeVersion = file('src/main/resources/minimumRuntimeVersion').text.trim()
+  targetCompatibility = minimumRuntimeVersion
+  sourceCompatibility = minimumRuntimeVersion
+}
+dependencies {
+  compile sourceSets.minimumRuntime.output
+  minimumRuntimeCompile "junit:junit:${props.getProperty('junit')}"
+  minimumRuntimeCompile localGroovy()
+}
+jar {
+  from sourceSets.minimumRuntime.output
+}
+
+
 /*****************************************************************************
  *                    Dependencies used by the entire build                  *
  *****************************************************************************/
@@ -94,10 +123,7 @@ dependencies {
   compile localGroovy()
   compile "com.carrotsearch.randomizedtesting:junit4-ant:${props.getProperty('randomizedrunner')}"
   compile "com.carrotsearch.randomizedtesting:randomizedtesting-runner:${props.getProperty('randomizedrunner')}"
-
-  compile("junit:junit:${props.getProperty('junit')}") {
-    transitive = false
-  }
+
   compile 'com.netflix.nebula:gradle-extra-configurations-plugin:3.0.3'
   compile 'com.netflix.nebula:nebula-publishing-plugin:4.4.4'
   compile 'com.netflix.nebula:gradle-info-plugin:3.0.3'
@@ -156,6 +182,7 @@ if (project != rootProject) {
   dependenciesInfo.enabled = false
   forbiddenApisMain.enabled = false
   forbiddenApisTest.enabled = false
+  forbiddenApisMinimumRuntime.enabled = false
   jarHell.enabled = false
   thirdPartyAudit.enabled = false