Set target and source compatibility to 11, required by Lucene 9.

Signed-off-by: dblock <dblock@dblock.org>

DEVELOPER_GUIDE.md

@@ -61,17 +61,33 @@ Fork [opensearch-project/OpenSearch](https://github.com/opensearch-project/OpenS
 
 #### JDK 11
 
-OpenSearch builds using Java 11 at a minimum. This means you must have a JDK 11 installed with the environment variable `JAVA_HOME` referencing the path to Java home for your JDK 11 installation, e.g. `JAVA_HOME=/usr/lib/jvm/jdk-11`.
+OpenSearch builds using Java 11 at a minimum, using the Adoptium distribution. This means you must have a JDK 11 installed with the environment variable `JAVA_HOME` referencing the path to Java home for your JDK 11 installation, e.g. `JAVA_HOME=/usr/lib/jvm/jdk-11`. This is configured in [buildSrc/build.gradle](buildSrc/build.gradle).
+
+```
+allprojects {
+  targetCompatibility = 11
+  sourceCompatibility = 11
+}
+```
 
 Download Java 11 from [here](https://adoptium.net/releases.html?variant=openjdk11).
 
 #### JDK 14
 
 To run the full suite of tests, download and install [JDK 14](https://jdk.java.net/archive/) and set `JAVA11_HOME`, and `JAVA14_HOME`. They are required by the [backwards compatibility test](./TESTING.md#testing-backwards-compatibility).
 
-#### Runtime JDK
+#### JDK 17
+
+By default, the test tasks use bundled JDK runtime, configured in [buildSrc/version.properties](buildSrc/version.properties), and set to JDK 17 (LTS).
+
+```
+bundled_jdk_vendor = adoptium
+bundled_jdk = 17.0.2+8
+```
+
+#### Custom Runtime JDK
 
-By default, the test tasks use bundled JDK runtime, configured in `buildSrc/version.properties` and set to JDK 17 (LTS). Other kind of test tasks (integration, cluster, ... ) use the same runtime as `JAVA_HOME`. However, the build supports compiling with JDK 11 and testing on a different version of JDK runtime. To do this, set `RUNTIME_JAVA_HOME` pointing to the Java home of another JDK installation, e.g. `RUNTIME_JAVA_HOME=/usr/lib/jvm/jdk-14`. Alternatively, the runtime JDK version could be provided as the command line argument, using combination of `runtime.java=<major JDK version>` property and `JAVA<major JDK version>_HOME` environment variable, for example `./gradlew -Druntime.java=17 ...` (in this case, the tooling expects `JAVA17_HOME` environment variable to be set).
+Other kind of test tasks (integration, cluster, etc.) use the same runtime as `JAVA_HOME`. However, the build also supports compiling with one version of JDK, and testing on a different version. To do this, set `RUNTIME_JAVA_HOME` pointing to the Java home of another JDK installation, e.g. `RUNTIME_JAVA_HOME=/usr/lib/jvm/jdk-14`. Alternatively, the runtime JDK version could be provided as the command line argument, using combination of `runtime.java=<major JDK version>` property and `JAVA<major JDK version>_HOME` environment variable, for example `./gradlew -Druntime.java=17 ...` (in this case, the tooling expects `JAVA17_HOME` environment variable to be set).
 
 #### Windows
 

buildSrc/build.gradle

@@ -158,8 +158,8 @@ if (project != rootProject) {
   apply plugin: 'opensearch.publish'
 
   allprojects {
-    targetCompatibility = 8
-    sourceCompatibility = 8
+    targetCompatibility = 11
+    sourceCompatibility = 11
   }
 
   // groovydoc succeeds, but has some weird internal exception...