Replace JSR-305 annotations with spotbugs annotations

Annotations for Nonnull, CheckForNull, and several others were proposed
for Java as part of dormant Java specification request JSR-305. The
proposal never became a part of standard Java.

Jenkins plugins should switch from using JSR-305 annotations to use
Spotbugs annotations that provide the same semantics.

The [mailing list discussion](https://groups.google.com/g/jenkinsci-dev/c/uE1wwtVi1W0/m/gLxdEJmlBQAJ)
from James Nord describes the affected annotations and why they should
be replaced with annotations that are actively maintained.

The ["Improve a plugin" tutorial](https://www.jenkins.io/doc/developer/tutorial-improve/replace-jsr-305-annotations/)
provides instructions to perform this change.

An [OpenRewrite recipe](https://docs.openrewrite.org/recipes/jenkins/javaxannotationstospotbugs)
is also available and is even better than the tutorial.

Confirmed that automated tests pass on Linux with Java 21.

src/main/java/com/coravy/hudson/plugins/github/GithubLinkAnnotator.java

@@ -9,9 +9,9 @@
 import hudson.scm.ChangeLogSet.Entry;
 import org.apache.commons.lang.StringUtils;
 
-import javax.annotation.CheckForNull;
-import javax.annotation.CheckReturnValue;
-import javax.annotation.Nonnull;
+import edu.umd.cs.findbugs.annotations.CheckForNull;
+import edu.umd.cs.findbugs.annotations.CheckReturnValue;
+import edu.umd.cs.findbugs.annotations.NonNull;
 
 import static hudson.Functions.htmlAttributeEscape;
 import static java.lang.String.format;
@@ -100,13 +100,13 @@ void process(MarkupText text, String url) {
  "(?:C|c)lose(?:s?)\\s(?<!\\:)(?:#)NUM", // "Closes #123"
  "issues/$1")};
 
- @Nonnull
+ @NonNull
  public static String getAllowedUriSchemes() {
  return StringUtils.join(ALLOWED_URI_SCHEMES, ',');
  }
 
  @CheckReturnValue
- @Nonnull
+ @NonNull
  public static boolean verifyUrl(@CheckForNull String urlString) {
  if (StringUtils.isBlank(urlString)) {
  return false;