Support building SDK with JVM 11

[rorbech]

Closes #7533
Closes #7179
Closes #7474 - Separated the actual fix of this into it's own PR in #7525

TODO

• 1. @rorbech merges Fix crash for cross module model definitions #7525 to master and then back into Support building SDK with JVM 11 #7535
• 2. @clementetb upgrades the project so it can compile with Java 11, i.e. fix Annotation Processor / CI.
  This also include doing work on CI so we e.g. compile the introExample with both Java 8 and Java 11 to catch future regressions. Compile with JDK11 #7539
  • Use JDK 11 on CI
  • Fix Javadoc generation
  • Fix jni header generation. JDK 11 requires new CMake construct for generating the headers (https://cmake.org/cmake/help/latest/module/UseJava.html#header-generation)
  • Include building at least one example project (introExample?) with both JDK 8 and JDK 11 on CI.
• 3. XXX upgrades our own repo to Gradle 7.11 and AGP to 7.0.0 and fix resulting issues (there seems to be a few). We need to take special care that our Gradle Plugin / Transformer / Annotation Processor still work with pre-7 and post-7 Gradle/AGP Update Gradle to version 7.1.1 #7548
• 4. We fix whatever needs fixing in the Gradle Plugin so we work with the configuration cache: Incompatibility issue with Gradle configuration cache #7299 in Support building SDK with JVM 11 #7535
• 7. For Kotlin, I suggest we just force people into AGP 7.0/Gradle 7/Java 11 and treat that as our minVersion.

[cmelchior]

Upgrading might be problematic due to https://issuetracker.google.com/issues/187134648. I have not yet been able to determine if there is a work-around.

[cmelchior]

Currently blocked on https://issuetracker.google.com/issues/202985154

[cmelchior]

Finally, ready for review. I disabled us building for Java 8 for now, as fixing that will grow this PR even more, and it is already kinda too big.

Note. Most changes to the realm-transformer was introduced and reviewed already in #7566

[clementetb]

LGTM, except that AS fails to build on task: :realm-library:buildCMakeRelWithDebInfo[x86_64][realm-jni].

The build mode RelWithDebInfo messes CMakeLists include directory path.

AS 2021.1.1 (bumblebee) is now required.

[clementetb]

Great! 🚀