PMM-7 Update default RPM build vars

[ademidoff]

PMM-7

This PR:

• updates the default RPM build variables after we stopped building RHEL7
• removes the redundant Dockerfile(s) for RHEL7-based rpmbuild images

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.25%. Comparing base (6712d56) to head (2c7ffe3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3245      +/-   ##
==========================================
+ Coverage   43.22%   43.25%   +0.02%     
==========================================
  Files         399      399              
  Lines       41725    41725              
==========================================
+ Hits        18037    18047      +10     
+ Misses      21680    21667      -13     
- Partials     2008     2011       +3     

Flag	Coverage Δ
admin	11.61% <ø> (ø)
agent	52.24% <ø> (+0.13%)	⬆️
managed	44.43% <ø> (-0.01%)	⬇️
vmproxy	66.66% <ø> (ø)

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