Bump maven-gpg-plugin from 1.4 to 3.1.0 (#259)

Bumps [maven-gpg-plugin](https://github.com/apache/maven-gpg-plugin) from 1.4 to 3.1.0.
- [Commits](apache/maven-gpg-plugin@maven-gpg-plugin-1.4...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>
Co-authored-by: Taro L. Saito <leo@xerial.org>

pom.xml

@@ -529,7 +529,7 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-gpg-plugin</artifactId>
-            <version>1.4</version>
+            <version>3.1.0</version>
             <executions>
               <execution>
                 <id>sign-artifacts</id>