fix: Skip Stratis tests on RHEL 8 and document Stratis support

[vojtechtrefny]

Unfortunately Blivet supports only the Stratis v3 API so we cannot support Stratis on RHEL/CentOS 8 where only Stratis v2 is available.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.73%. Comparing base (59fd1c6) to head (bfff8f9).
Report is 2 commits behind head on main.

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HEAD has 1 upload less than BASE

Flag	BASE (59fd1c6)	HEAD (bfff8f9)
sanity	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #482      +/-   ##
==========================================
- Coverage   16.54%   10.73%   -5.81%     
==========================================
  Files           2        8       +6     
  Lines         284     1946    +1662     
  Branches       79        0      -79     
==========================================
+ Hits           47      209     +162     
- Misses        237     1737    +1500     

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[vojtechtrefny]

[citest]