Merge pull request #18047 from rjernst/javac_target

Build: Explicitly set target and source compatibility for javac

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

@@ -355,6 +355,11 @@ class BuildPlugin implements Plugin<Project> {
                 }
                 options.encoding = 'UTF-8'
                 //options.incremental = true
+
+                // gradle ignores target/source compatibility when it is "unnecessary", but since to compile with
+                // java 9, gradle is running in java 8, it incorrectly thinks it is unnecessary
+                assert minimumJava == JavaVersion.VERSION_1_8
+                options.compilerArgs << '-target' << '1.8' << '-source' << '1.8'
             }
         }
     }