[Hacktoberfest] Ensure the code compiled with java 11 and find places to use new API's from java 11.

**Situation**
We migrated the app from java 8 to java 11, on: https://github.com/commercetools/commercetools-project-sync/releases/tag/3.12.0

**Complication**
As we could use java 11, we might benefit of new apis from java 11 in the project.
Also we are not sure if the project is compiling with java 11:
https://github.com/commercetools/commercetools-project-sync/blob/90bfbfa11d07ae3d7e017d57da815cafaa96f0d5/gradle-scripts/java-compile.gradle#L1-L6

**Resolution**
Ensure the code compiled with java 11 and find places to use API's from java 11. If possible refactor code to use new APIs from java 11.