geant4: new version 10.6 plus simplifications (#15447)

* geant4: new version 10.6 plus simplifications

Add new 10.6.0 release, migrating download of source to use Geant4's
public release repo on CERN GitLab. Change versioning scheme to use
clearer and standard semantic scheme.

Update geant4-data and g4XXX data packages with new versions. Migrate
geant4-data as a BundlePackage of the g4XXX packages, installing links
to each under a single directory under share for geant4-data. Ensure
each g4XXX package exports the environment variable pointing to its
location expected by Geant4.

Remove "data" variant from Geant4 package and always use geant4-data.

Simplify cxxstd variant transport to dependencies.

* g4<DATA>: Use self to resolve correct prefix

* geant4, data: Fix flake8 errors

* g4photonevaporation: flake8 fix

* geant4: vecgeom version depends_on

Geant4 major.minor versions have specific dependencies on vecgeom
versions. Add missing vecgeom version for geant4 10.5, and match
version requirements for vecgeom in geant4 depends_on.

* geant4: c++17 patch specific for 10.4.3

* geant4: simplify geant4-data setup

* geant4: Use new define_from_variant function

* geant4: fix flake8 errors

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