Use spotbugs annotations instead of javax annotations

The parent pom provides spotbugs reports by default.  Let's use spotbugs
annotations because they are used in other Jenkins plugins and in
Jenkins core.

See https://issues.jenkins.io/browse/JENKINS-55973 and
jenkinsci/jenkins#4604

src/main/java/jenkins/branch/buildstrategies/basic/AllBranchBuildStrategyImpl.java

@@ -43,7 +43,7 @@
 import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.DataBoundConstructor;
 
-import javax.annotation.CheckForNull;
+import edu.umd.cs.findbugs.annotations.CheckForNull;
 
 /**
  * A {@link BranchBuildStrategy} that builds branches based on the results of all sub strategies matching.

src/main/java/jenkins/branch/buildstrategies/basic/AnyBranchBuildStrategyImpl.java

@@ -36,7 +36,7 @@
 import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.DataBoundConstructor;
 
-import javax.annotation.CheckForNull;
+import edu.umd.cs.findbugs.annotations.CheckForNull;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;

src/main/java/jenkins/branch/buildstrategies/basic/BranchBuildStrategyImpl.java

@@ -37,7 +37,7 @@
 import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.DataBoundConstructor;
 
-import javax.annotation.CheckForNull;
+import edu.umd.cs.findbugs.annotations.CheckForNull;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 

src/main/java/jenkins/branch/buildstrategies/basic/NamedBranchBuildStrategyImpl.java

@@ -38,7 +38,7 @@
 import java.util.logging.Logger;
 import java.util.regex.Pattern;
 import java.util.regex.PatternSyntaxException;
-import javax.annotation.Nonnull;
+import edu.umd.cs.findbugs.annotations.NonNull;
 
 import hudson.util.LogTaskListener;
 import jenkins.branch.BranchBuildStrategy;
@@ -249,7 +249,7 @@ public String toString() {
         @Symbol("exact")
         @Extension
         public static class DescriptorImpl extends NameFilterDescriptor {
-            @Nonnull
+            @NonNull
             @Override
             public String getDisplayName() {
                 return Messages.NamedBranchBuildStrategyImpl_exactDisplayName();
@@ -323,7 +323,7 @@ public String toString() {
         @Symbol("regex")
         @Extension
         public static class DescriptorImpl extends NameFilterDescriptor {
-            @Nonnull
+            @NonNull
             @Override
             public String getDisplayName() {
                 return Messages.NamedBranchBuildStrategyImpl_regexDisplayName();
@@ -462,7 +462,7 @@ private String getPattern(String names) {
         @Symbol("wildcards")
         @Extension
         public static class DescriptorImpl extends NameFilterDescriptor {
-            @Nonnull
+            @NonNull
             @Override
             public String getDisplayName() {
                 return Messages.NamedBranchBuildStrategyImpl_wildcardDisplayName();

src/main/java/jenkins/branch/buildstrategies/basic/NoneBranchBuildStrategyImpl.java

@@ -36,7 +36,7 @@
 import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.DataBoundConstructor;
 
-import javax.annotation.CheckForNull;
+import edu.umd.cs.findbugs.annotations.CheckForNull;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;

src/main/java/jenkins/branch/buildstrategies/basic/SkipInitialBuildOnFirstBranchIndexing.java

@@ -35,7 +35,7 @@
 import org.kohsuke.accmod.restrictions.NoExternalUse;
 import org.kohsuke.stapler.DataBoundConstructor;
 
-import javax.annotation.CheckForNull;
+import edu.umd.cs.findbugs.annotations.CheckForNull;
 
 
 @Restricted(NoExternalUse.class)

src/test/java/jenkins/branch/buildstrategies/basic/harness/BasicBranchProjectFactory.java

@@ -31,7 +31,6 @@
 import hudson.model.TaskListener;
 import java.io.IOException;
 import java.util.Map;
-import javax.annotation.Nonnull;
 import jenkins.branch.MultiBranchProject;
 import jenkins.branch.MultiBranchProjectFactory;
 import jenkins.branch.MultiBranchProjectFactoryDescriptor;