Target .NET6 instead of .NET Standard 2.0 and .NET Core 3.1

[Mafii]

@natemcmaster to ease reviewing, here's a PR that just changes the target framework. I have not taken a look at your CI scripts, so there may be some work required for them. (edit: they seem to work)

A PR for the other feature will follow :)

The review is probably easier on a per-commit basis, I tried to make them as small and concise as possible, especially trying to separate gardening and actual changes.

[codecov[bot]]

Codecov Report

Merging #280 (c76dd2f) into main (f31dbfa) will decrease coverage by 0.38%. The diff coverage is 48.27%.

@@            Coverage Diff             @@
##             main     #280      +/-   ##
==========================================
- Coverage   46.16%   45.77%   -0.39%     
==========================================
  Files          45       45              
  Lines        1120     1112       -8     
==========================================
- Hits          517      509       -8     
  Misses        603      603              

Impacted Files	Coverage Δ
...ncrypt.Azure/AzureKeyVaultLettuceEncryptOptions.cs	100.00% <ø> (ø)
.../LettuceEncrypt/Internal/AcmeCertificateFactory.cs	0.00% <ø> (ø)
...c/LettuceEncrypt/Internal/AcmeCertificateLoader.cs	0.00% <0.00%> (ø)
src/LettuceEncrypt/Internal/AcmeClient.cs	0.00% <0.00%> (ø)
...ternal/AcmeStates/BeginCertificateCreationState.cs	0.00% <0.00%> (ø)
...ternal/DefaultCertificateAuthorityConfiguration.cs	100.00% <ø> (ø)
src/LettuceEncrypt/Internal/DeveloperCertLoader.cs	94.73% <ø> (ø)
...ettuceEncrypt/Internal/DomainOwnershipValidator.cs	0.00% <ø> (ø)
.../LettuceEncrypt/Internal/FileSystemAccountStore.cs	95.12% <ø> (ø)
...c/LettuceEncrypt/Internal/Http01DomainValidator.cs	0.00% <0.00%> (ø)
... and 16 more

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[natemcmaster]

https://github.com/natemcmaster/LettuceEncrypt/releases/tag/v1.3.0-beta.249