chore(deps): bump com.amashchenko.maven.plugin:gitflow-maven-plugin

Bumps [com.amashchenko.maven.plugin:gitflow-maven-plugin](https://github.com/aleksandr-m/gitflow-maven-plugin) from 1.19.0 to 1.21.0.
- [Release notes](https://github.com/aleksandr-m/gitflow-maven-plugin/releases)
- [Changelog](https://github.com/aleksandr-m/gitflow-maven-plugin/blob/master/CHANGELOG.md)
- [Commits](aleksandr-m/gitflow-maven-plugin@v1.19.0...v1.21.0)

---
updated-dependencies:
- dependency-name: com.amashchenko.maven.plugin:gitflow-maven-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -21,7 +21,7 @@
         <!-- Maven Plugins -->
         <maven-release-plugin.version>2.5.1</maven-release-plugin.version>
         <maven-site-plugin.version>3.3</maven-site-plugin.version>
-        <plugin.gitflow-maven.version>1.19.0</plugin.gitflow-maven.version>
+        <plugin.gitflow-maven.version>1.21.0</plugin.gitflow-maven.version>
     </properties>
 
     <licenses>