Adapt to the latest Acts developments (#16385)

* Adapt to the latest Acts developments

A long time ago, the Acts project (whose name was then capitalized ACTS) used
to maintain multiple software repositories:

- The heart of the tracking toolkit was located in the `acts-core` repository
- Fast simulation extensions were located in the `acts-fatras` repository
- Advanced usage examples were located in the `acts-framework` repository

This multi-repository organization, however, has been a source of constant
pain, which is why the various projects were gradually merged into a single
mono-repo, called `acts`. Today, with the integration of `acts-framework`,
this merging process is reaching completion.

The present pull request adapts the Acts package to this evolution by...

- Renaming the package to `acts`, reflecting the new repository name
- Renaming the `test` variant to `unit_tests`, reflecting current CMake naming
- Adding the new build variants that were inherited from `acts-framework`
- Acknowledging the change of semantics of the `examples` variant, and only
  supporting the new ones (as the former variant was almost unused)
- Liberally using alphabetical order to make the package code more readable
- Recording a large number of conflicts, some of which are introduced by the
  merging of `acts-framework` and some of which already existed before
- Using the new capitalization of "Acts"

* Add acts v0.23

* Update dd4hep version requirement

* Add acts v0.22.1 bugfix

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