Update code to Java 11

[nipafx]

A first stab ab #706.

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Documentation (general)

• There is documentation (Javadoc and site documentation; added or updated)
• There is implementation information to describe why a non-obvious source code / solution got implemented
• Site documentation has its own .adoc file in the docs folder, e.g. docs/report-entries.adoc
• Site documentation in .adoc file references demo in src/demo/java instead of containing code blocks as text
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Documentation (new extension)

• The docs/docs-nav.yml navigation has an entry for the new extension
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Contributing

• A prepared commit message exists
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[Bukama]

I would like to see that we write valid HTML (see comment on "p"-Tag, applies on many changes in the PR)

[Bukama]

One more closing paragraph tag and I'm fine with it (if you added it, see this an approval and feel free to merge, as @beatngu13 already accepted)

[Michael1993]

Proposed commit message:

Update code to Java 11 (#706 / #707)

This commit updates the code base to use the language features now
available to us (as we baseline against Java 11).

PR: #707

[Bukama]

Thank you for your patience :)