Drop support for .NET 6

[martincostello]

#2798 drops support for most out-of-support versions of .NET, but to give users time to react, version 7.0.0 retains support for net6.0 even though by the time it ships .NET 6 will be out of support.

We should drop support for net6.0 in a release some time afterwards.

See also #2798 (comment).

[github-actions]

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