fix(deps): Remove redundant TFM-specific builds

[bartelink]

potential follow-up to #184 See https://github.com/serilog/serilog/issues/1970 for lots of reasoning, but the TL;DR is:

In the general case, a library should depend on the minimum TFM that it can be implemented in terms of. For many libs, thats netstandard2.0, and supporting that is great when it's possible.

• In the netstandard2.1 - net5.0 timeframe, there are various confusions, and ultimately there's not benefit to specifically targeting them or thinking about them

• net6.0 brings significant available API surface and removes the need for complex conditional dependencies

Adding TFM-specific builds for stuff later than net6.0 on the other hand brings little concrete benefit. i.e.

• builds take longer
• nupkg is larger
• more analysis required to figure out the TFM flavor being used in a given runtime scenario
• in very rare cases, if the compiler knows that the TFM is recent, it can lean on newer intrinsics, which can confer a size or speed advantage
• people see that net6.0, net7.0 and net8.0 are supported but agonize needlessly about whether net9.0 is "supported"

[bartelink]

I'm definitely not trying to push this one as aggressively as 184, but I am pretty confident that there's nothing to be gained by having a net7.0 or net8.0 build of the package

Obviously if it's possible to achieve more or improve performance by targeting an API only present in a newer TFM, then there's a choice to be made between:

• up the min requirement from net6.0
• add a newer TFM-specific build, and use conditional compilation to do that new stuff only for the newer TFM

My objection is to making life more complex than it needs to be until such time as there's something in a newer TFM that's relevant to the task at hand.