Reduce max javac memory to 512mb (#28783)

With this commit we reduce the maximum amount of memory that the javac
compiler can use from 1g to 512mb. While the build would succeed even
with 256mb, it influences compile time slightly negatively.

We have measured that the runtime overhead stays tolerable by running
the following command five times under repeatable conditions (i.e. we
execute `./gradlew clean`, then drop the caches and TRIM the disk):

```
./gradlew compileGroovy compileJava compileJava9Java\
 compileTestGroovy compileTestJava compileGroovy
```

The results in seconds (as reported by Gradle) are:

* 1gb: avg: 253s, min: 252s, max: 256s
* 512mb: avg: 257s, min: 256s, max: 259s

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

@@ -430,7 +430,7 @@ class BuildPlugin implements Plugin<Project> {
                 // we fork because compiling lots of different classes in a shared jvm can eventually trigger GC overhead limitations
                 options.fork = true
                 options.forkOptions.javaHome = new File(project.compilerJavaHome)
-                options.forkOptions.memoryMaximumSize = "1g"
+                options.forkOptions.memoryMaximumSize = "512m"
                 if (targetCompatibilityVersion == JavaVersion.VERSION_1_8) {
                     // compile with compact 3 profile by default
                     // NOTE: this is just a compile time check: does not replace testing with a compact3 JRE