Raise minimal Java version to 17

[kthoms]

Is your feature request related to a problem? Please describe. Current minimal version is 11. This should be raised. See the discussion in the forum.

Describe the solution you'd like

• Configure enforcer plugin to require Java 17
• Change compiler properties in root pom.xml
• Ensure that the CI build uses at least Java 17
• Remove redundant maven-enforcer-plugin configs from submodules
• Document prerequisite in README

Describe alternatives you've considered Leaving lower bound 11, or upgrade to current LTS release 21.

[kthoms]

The build workflow already uses Java 17.

Configuration of Java 17 in the enforcer configuration in the root pom.xml makes several configuration in submodules obsolete.