refactor: Add TFMs for net{6,7,8}.0

[austindrenski]

As a consumer of the OpenFeature SDK, I want OpenFeature.dll to be built and tested against all of the supported TFMs that my projects might encounter in the wild.

This PR:

1. Updates the global.json to 8.0.100
2. Updates workflows using only 7.0.x to instead resolve based on global.json
3. Updates the project(s) TFMs to include net6.0, net7.0, and net8.0
   • net6.0 is EOL in 2024-11-12
   • net7.0 is EOL in 2024-05-14

Fixes: #160, #169

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c1a189a) 94.73% compared to head (f93659a) 94.73%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #172   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
=======================================
  Files          23       23           
  Lines         931      931           
  Branches       93       93           
=======================================
  Hits          882      882           
  Misses         29       29           
  Partials       20       20           

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[benjiro]

I'm fine with this probably a bigger question currently we aren't really using any features above .netstandard2.0 so targeting net6,7,8 is really pointless because there is no runtime benefit. I guess in the future we can look at optimisations and utilize some of the new runtime feature were available