Bump maven-gpg-plugin from 1.6 to 3.1.0 (cloudevents#564)

Bumps [maven-gpg-plugin](https://github.com/apache/maven-gpg-plugin) from 1.6 to 3.1.0.
- [Commits](apache/maven-gpg-plugin@maven-gpg-plugin-1.6...maven-gpg-plugin-3.1.0)

---
updated-dependencies:
- dependency-name: org.apache.maven.plugins:maven-gpg-plugin
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Signed-off-by: Alex Collins <alex_collins@intuit.com>

pom.xml

@@ -244,7 +244,7 @@
                     <plugin>
                         <groupId>org.apache.maven.plugins</groupId>
                         <artifactId>maven-gpg-plugin</artifactId>
-                        <version>1.6</version>
+                        <version>3.1.0</version>
                         <executions>
                             <execution>
                                 <id>sign-artifacts</id>