:breaking-change: bump version to 5.0 and drop support for .NET Standard 2 and < .NET 6

[natemcmaster]

This change makes the following breaking changes:

• Drops support for .NET Standard 2
• Drops support for any version of .NET < 6.0
• Removes some internal code that was necessary to preserve backwards compat with Mono and .NET Framework 4

The major version of this package is bumped to 5.0 to indicate this may break for some users.

[codecov]

Codecov Report

Attention: Patch coverage is 64.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 77.79%. Comparing base (10d0155) to head (5041bc6).

Files	Patch %	Lines
src/CommandLineUtils/Properties/Strings.cs	12.50%	1 Missing and 6 partials ⚠️
...mandLineUtils/Attributes/AllowedValuesAttribute.cs	0.00%	0 Missing and 1 partial ⚠️
...ommandLineUtils/Validation/ValidationExtensions.cs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
- Coverage   77.85%   77.79%   -0.07%     
==========================================
  Files         105      104       -1     
  Lines        3310     3319       +9     
  Branches      716      722       +6     
==========================================
+ Hits         2577     2582       +5     
+ Misses        580      578       -2     
- Partials      153      159       +6     

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