Improvement: Expose T8CODE_VTK_VERSION_USED and T8_VTK_VERSION_USED to linking libraries.

[holke]

Closes #2100

Describe your changes here:

Follow up to #2012 and related to #2008

Additionally to making external libraries aware of t8code being linked against VTK with #2012. we now also export the macros
T8CODE_VTK_VERSION_USED (now available in CMake scripts of external libraries) and
T8_VTK_VERSION_USED (now available in source code of external libraries).

Both have the same value.

(We could also have one name for both. I sticked to the existing naming scheme here and did both variants.)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (63de7bd) to head (741ffdf).
⚠️ Report is 1 commits behind head on fix_vtk_linkage.

Additional details and impacted files

@@               Coverage Diff                @@
##           fix_vtk_linkage    #2099   +/-   ##
================================================
  Coverage            77.46%   77.46%           
================================================
  Files                  112      112           
  Lines                19084    19084           
================================================
  Hits                 14784    14784           
  Misses                4300     4300           

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