Require JDK 10 to build Elasticsearch (#29174)

This commit bumps the minimum compiler version required to build
Elasticsearch from JDK 9 to JDK 10.

CONTRIBUTING.md

@@ -92,11 +92,11 @@ Contributing to the Elasticsearch codebase
 
 **Repository:** [https://github.com/elastic/elasticsearch](https://github.com/elastic/elasticsearch)
 
-JDK 9 is required to build Elasticsearch. You must have a JDK 9 installation
+JDK 10 is required to build Elasticsearch. You must have a JDK 10 installation
 with the environment variable `JAVA_HOME` referencing the path to Java home for
-your JDK 9 installation. By default, tests use the same runtime as `JAVA_HOME`.
+your JDK 10 installation. By default, tests use the same runtime as `JAVA_HOME`.
 However, since Elasticsearch, supports JDK 8 the build supports compiling with
-JDK 9 and testing on a JDK 8 runtime; to do this, set `RUNTIME_JAVA_HOME`
+JDK 10 and testing on a JDK 8 runtime; to do this, set `RUNTIME_JAVA_HOME`
 pointing to the Java home of a JDK 8 installation. Note that this mechanism can
 be used to test against other JDKs as well, this is not only limited to JDK 8.
 

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -58,7 +58,7 @@ import java.time.ZonedDateTime
 class BuildPlugin implements Plugin<Project> {
 
     static final JavaVersion minimumRuntimeVersion = JavaVersion.VERSION_1_8
-    static final JavaVersion minimumCompilerVersion = JavaVersion.VERSION_1_9
+    static final JavaVersion minimumCompilerVersion = JavaVersion.VERSION_1_10
 
     @Override
     void apply(Project project) {