Fix problems building on JDK 11

[zambrovski]

Currently, the extension is built using JDK 11 and JDK 8 on GH.
The dokka plugin is not working JDK 11 (as far as I know), so the default for building should be JDK8, if we build dokka pages, or even better the creation of sources and javadoc should be moved to a separate profile used by release process only (which can then work on JDK 8).

[zambrovski]

The problem with current build is not related to JDK 8 / JDK 11, but with Kotlin 1.4.0 -> I reverted it back to 1.3.61 and then the type inference in query gateways works again...