Drop Java 8 support + Java 11 and Jenkins update + Migrate to Eclipse Temurin official images + Rename Master branch to Main

[oleg-nenashev]

A bulk update that removes support for Java 8 and updates dependencies. It also does a bulk update for demos accordingly.

Why? Currently Jenkinsfile Runner base images are distributed with Java 8 by default. Also, there is ongoing work on Java 17 support in Jenkins and Jenkinsfile Runner in https://github.com/jenkinsci/jenkinsfile-runner/issues/569, as well as removal of Java 8 support in the recent Jenkins releases.

• [x] Drop Java 1.8 support
• [x] Move Java 11 images to the recent Eclipse Temurin versions, including official Java 11 Alpine image
• [x] Update to the recent Jenkins core. I stick to LTS for now
• [x] Reflect master => Main renaming
• [x] Use GitHub registry as a main source of Docker images

Closes #584

[oleg-nenashev]

Now Spotbugs fails

[2022-08-08T13:42:53.492Z] [ERROR] Failed to execute goal com.github.spotbugs:spotbugs-maven-plugin:4.7.1.1:check (spotbugs) on project bootstrap: failed with 22 bugs and 0 errors -> [Help 1]

[2022-08-08T13:42:53.492Z]