Drop netcoreapp3.1 target

[iamcarbon]

This PR drops support for the netcoreapp3.1 target (now out of support).

.netcoreapp3.1 will get the netstandard2.1 target after this update.

[coveralls]

Pull Request Test Coverage Report for Build 3718428439

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.292%

---

Totals
Change from base Build 3614357950:	0.0%
Covered Lines:	25918
Relevant Lines:	27173

---

💛 - Coveralls

[omer-iqbal]

Hi,

It might also help readers if the readme.md at root is updated which lists the compatible frameworks.

Thx

[iamcarbon]

@omer-iqbal The library continues to support ALL supported .NET frameworks -- and this PR does not change that. The netstandard2.1 target allows the library to continue to be used by netcoreapp3.1.

Nuget does a great job at showing the libraries compatibility: https://www.nuget.org/packages/Markdig

[xoofx]

Thanks!