Add gradle jvmargs to allow spotlessApply to work with newer Java. (triplea-game#10209)

Without this, running spotlessApply hits an error.

See diffplug/spotless#834.

Author: asvitkine

gradle.properties

@@ -1 +1,6 @@
 org.gradle.caching=true
+org.gradle.jvmargs=--add-exports jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED