Maintenance: lock down Maven

Changes:
* use Wrapper and 3.9.12
* make sure CI uses wrapper (we control Maven version not the GH action)
* use `release` compiler flag instead of `source`/`target` to set 11 level
* enable reproducible build

Author: cstamas

.github/workflows/ci.yml

@@ -24,4 +24,4 @@ jobs:
     - name: Install and run IPFS
       run: ./install-run-ipfs.sh
     - name: Build & run tests with Maven
-      run: mvn test
+      run: ./mvnw verify

.mvn/wrapper/maven-wrapper.properties

@@ -0,0 +1,3 @@
+wrapperVersion=3.3.4
+distributionType=only-script
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.12/apache-maven-3.9.12-bin.zip