feat! add Temurin JDK11 11.0.20+8 as default JDK (breaking)

[dduportal]

74 switched the default JDK11 (from the Jenkins agent image) to JDK17.

But (at least for today) Jenkins is built with JDK11 v11.0.20 so we need to keep the same JDK for now.

This PR adds back the JDK11 by separating explicitly the JDK from agent and the JDK for building.

The JDK for the agent is NOT added as the default in update-aleternatives as the JAVA_HOME is used for the build JDK e.g. the 11: this is a breaking change.

Note:

• No updatecli manifest added in this PR (in a subsequent one).

[dduportal]

Result of mvn -v after this change:

Apache Maven 3.8.6 (84538c9988a25aec085021c365c560670ad80f63)
Maven home: /usr/share/apache-maven-3.8.6
Java version: 11.0.20, vendor: Eclipse Adoptium, runtime: /opt/jdk-11
Default locale: en, platform encoding: UTF-8