Make sure right protoc version is applied

The protoc-jar-maven-plugin currently is not applying our configured protoc version, but instead falls back to its own default version of protoc. The reason is that the protoc version property is defined in the BOM POM and thus isn't visible to the Postgres connector module which relies on the BOM via import scope.

Noticed this btw. while building on macOS/AArch64, which isn't supported by the earlier protoc version, so my build would fail due to that.