Replace JSR-305 annotation with spotbugs annotation

Details at jenkinsci/jenkins#4604

See also https://issues.jenkins.io/browse/JENKINS-55973

https://docs.google.com/document/d/1PKYIpPlRVGsBqrz0Ob1Cv3cefOZ5j2xtGZdWs27kLuw/edit#heading=h.vxl6or38wd08

Author: MarkEWaite

src/main/java/org/jenkinsci/plugins/gogs/GogsPayload.java

@@ -5,7 +5,8 @@
 import hudson.model.EnvironmentContributingAction;
 import hudson.model.InvisibleAction;
 
-import javax.annotation.Nonnull;
+import edu.umd.cs.findbugs.annotations.NonNull;
+
 import java.util.Map;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -17,7 +18,7 @@ public GogsPayload(Map<String, String> payload) {
         this.payload = payload;
     }
 
-    @Nonnull
+    @NonNull
     private Map<String, String> getPayload() {
         return payload;
     }
@@ -32,4 +33,4 @@ public void buildEnvVars(AbstractBuild<?, ?> abstractBuild, EnvVars envVars) {
     }
 
     private static final Logger LOGGER = Logger.getLogger(GogsPayload.class.getName());
-}
+}