Upgrade to Maven 3.9.9

See gh-913

Author: mp911de

.mvn/wrapper/maven-wrapper.properties

@@ -1 +1 @@
-distributionUrl=https://repo1.maven.org/maven2/org/apache/maven/apache-maven/3.6.3/apache-maven-3.6.3-bin.zip
+distributionUrl=https://repo1.maven.org/maven2/org/apache/maven/apache-maven/3.9.9/apache-maven-3.9.9-bin.zip

ci/version.sh

@@ -3,7 +3,7 @@
 set -euo pipefail
 
 RAW_VERSION=`MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw \
-  org.apache.maven.plugins:maven-help-plugin:3.2.0:evaluate \
+  org.apache.maven.plugins:maven-help-plugin:3.5.1:evaluate \
   -Dexpression=project.version -q -DforceStdout`
 
 # Split things up