Upgrade to Error Prone 2.39.0

Closes gh-3

Author: wilkinsona

src/main/java/io/spring/gradle/nullability/NullabilityPluginExtension.java

@@ -25,7 +25,7 @@
  */
 public abstract class NullabilityPluginExtension {
 
-	static final String ERROR_PRONE_VERSION = "2.38.0";
+	static final String ERROR_PRONE_VERSION = "2.39.0";
 
 	static final String NULL_AWAY_VERSION = "0.12.7";