Align runtime requirements with EF packages

[TobbenTM]

An issue we found using this package is that as soon as you upgrade to EF 7.0 packages, but still running on .Net 6, it fails since it's referencing the wrong EF assemblies.

Specifically, we're seeing issues like this:

System.MissingMethodException: Method not found: 'System.Collections.Generic.IList`1<Microsoft.EntityFrameworkCore.Metadata.Conventions.IModelFinalizingConvention> Microsoft.EntityFrameworkCore.Metadata.Conventions.ConventionSet.get_ModelFinalizingConventions()'.
   at XO.EntityFrameworkCore.NpgsqlJsonSerializerOptions.JsonSerializerOptionsConventionSetPlugin.ModifyConventions(ConventionSet conventionSet)

To align this package with how EF packages are published (all compatible with .Net 6, even 7.0 packages), this PR removes the runtime conditional switch for dependencies.
In this move, support for EF 6 is also dropped, but I'd argue that anyone needing EF 6 compatibility could use a earlier version of this package, just like how they consume EF itself.

Would love to hear your thoughts on this, because we've found this package quite helpful while we await EF 8.0!

[wjrogers]

Sounds reasonable to me. I wanted to make the library compatible with both versions 6 and 7 of EF Core, but I suppose publishing different major versions would have been a better way of doing that. Thanks for contributing!

[codecov]

Codecov Report

Merging #3 (80d5386) into main (1f956de) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main       #3   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files          10       10           
  Lines         138      138           
=======================================
  Hits          130      130           
  Misses          8        8           

Components	Coverage Δ
XO.EntityFrameworkCore.NpgsqlJsonSerializerOptions	94.20% <ø> (ø)

[TobbenTM]

Yeah, having matching versions, maintaining different packages for 6.0 and 7.0 is probably "the most correct" approach, but also takes effort.
Anyways, with the latest 2.0.4 version you published with these changes, our app is happy again, able to do nice ser/de!
Thanks for the quick response here!