Release 6.0.0

[stakx]

• Renew NuGet API key on AppVeyor → @jonorossi
• Ensure CI build succeeds
• Review issues (are all issues assigned to the 6.0.0 milestone completed?)
• Update changelog (version and release date)
• Create GitHub release and Git version tag, then 🤞
• Verify packages are published to & listed on NuGet

[drauch]

Can't wait 🚀

[stakx]

Ideally, we would have finally been able to decide which components and APIs should be marked [Obsolete] or removed immediately (logging facilities, DictionaryAdapter, etc.)... but it seems that has to wait for the next major release.

I think what we have now is good to go.

[stakx]

Bad news, there's going to be another delay as the NuGet API key has expired again. I'll have to let @jonorossi resolve that issue.

[stakx]

Since 6.0.0 currently cannot be released anyway due to technical reasons, I've decided to untag v6.0.0 to give us some more time to resolve the ongoing discussion in #663.