Antora release upgrade from 2.3.4 to 3.0.x

[mmattel]

This issue is a placeholder for the new Anotora release.

I expected that the new Antora release is coming in Q1 or Q2 2022...
Now:
Antora 2.3 will enter maintenance on January 25, 2022 and reach end of life (EOL) on February 25, 2022.

Preperations have to be done like to

• move the _partials folder and rename it (_partials is depreciated and access will be/has been removed)
• look for changes in 3.0 which are not backwards compatible
• intense checks of the html build process
• pdf build check
• compatibility of extensions used (see site.yml)
• ...

Nothing that can be done in a view days.

Even more, such a change also affects all doc-xxx repos where this has to be in sync

#4543 (Bump @antora/cli from 2.3.4 to 3.0.0)
#4542 (Bump @antora/site-generator-default from 2.3.4 to 3.0.0)

Note that the referenced PR's must not be merged until the tests have been successfully finished.

See:
https://docs.antora.org/antora/latest/whats-new/
https://docs.antora.org/antora/latest/install/upgrade-antora/

@EParzefall @phil-davis fyi

[mmattel]

Refrencing issue: #4128

With #4128 we have harmonized over all repos the antora directory structure as it is recommended and necessary as preperation for Antora 3.x This also includes the helper scripts for finding orphands.

Before an upgrade can be made, the pdf generation must be fixed. The current way is not well embedded into the antora framework, needs dynamical generated attributes and has deprecated attributes accessing family resources.

When the Antora team provides the pdf generator officially, we can switch to the new generator and eliminate the issues described above. When this is done, we can set all necessary steps going for Antora 3.x

Here is a link for the upgrade notes: Asciidoctor Upgrade Notes
Note the Upgrade Antora for instructions section at the bottom