Install Maven using apt-get instead of downloading it manually

The Debian bullseye repo (and also buster-backports) ships Maven 3.6.3,
the same version that we downloaded manually, so it *should* work
without issues.

.gitlab-ci.yml

@@ -4,14 +4,8 @@ build:
   before_script:
     - export DEBIAN_FRONTEND="noninteractive"
     - apt-get update
-    # perl provides shasum
-    - apt-get install --assume-yes curl perl
-    - curl --location -o apache-maven-3.6.3-bin.tar.gz https://archive.apache.org/dist/maven/maven-3/3.6.3/binaries/apache-maven-3.6.3-bin.tar.gz
-    - shasum --algorithm 512 --quiet --check - <<< "c35a1803a6e70a126e80b2b3ae33eed961f83ed74d18fcd16909b2d44d7dada3203f1ffe726c17ef8dcca2dcaa9fca676987befeadc9b9f759967a8cb77181c0 *apache-maven-3.6.3-bin.tar.gz"
-    - tar -zxvf apache-maven-3.6.3-bin.tar.gz
+    - apt-get install --assume-yes maven
     - export MAVEN_OPTS="-Xmx512m -XX:MaxPermSize=192m"
-    - export M2_HOME=$PWD/apache-maven-3.6.3
-    - export PATH=$M2_HOME/bin:$PATH
     - mvn --version
   script:
     - ./gradlew createPoms