Remove net7.0 target

[nietras]

Upcoming changes/refactorings will require using UnsafeAccessor to make SepWriter simpler and more efficient which is only available in net8.0+ so dropping .NET 7 support. If any users of Sep have an issue with this please chime in in this PR and why can't upgrade to .NET 8 or similar.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (b0a2c59) to head (fa2107e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #218   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files          47       47           
  Lines        3602     3602           
  Branches      403      403           
=======================================
  Hits         3587     3587           
  Misses         13       13           
  Partials        2        2           

Flag	Coverage Δ
Debug	99.41% <ø> (ø)
Release	99.84% <ø> (ø)
macos-latest	92.00% <ø> (-0.20%)	⬇️
ubuntu-latest	99.84% <ø> (+0.28%)	⬆️
windows-latest	99.55% <ø> (ø)

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