* Clean up code style in Gradle configs
* Fix JavaDoc errors
* Make JUnit 4 as `optional` dependency for `spring-integration-test-support`
* Remove redundant `javax.activation-api` from some modules
* Move all the docs related configuration into a `docs.gradle`
* Move `publish-maven.gradle` into a `/gradle` dir for consistency
* Upgrade to Kotlin `1.4.30`
* Remove redundant `repository.apache.org` since we don't manage direct dependency for Kafka Client
* Upgrade to Gradle `6.8.2`
Resolves https://github.com/spring-projects/spring-integration/issues/3490
- also fix double import of `jmx.adoc`
- also fix `redis.adoc`
* Remove redundant repo in `docs.gradle`
* Use `stylesheet.css` for `asciidoctorj` plugin config
* Fix `stylesheet.css` to import `spring.css` from the same dir
since they both are copied over there into the same dir
* Fix link for JMX chapter from the `metrics.adoc`
* Fix some inconsistency in the `amqp.adoc`
