Bump version

Use JDK 18 runner and target 19

Improve Javadoc, fix headers, add proper checkstyle

Try running Gradle under JDK 11

Upgrade to RxJava 3.1.1

Release 0.0.13

Release 0.0.12

Upgrade to RxJava 3.0.0 final.

Upgrade to RxJava 3.0.0-RC8

Release 0.0.7