Use latest C# in BSATN.Runtime

[RReverser]

Description of Changes

Now that we include it by DLL rather than by source in the Unity SDK, we no longer need to limit ourselves to C# 9 - the only limitation is now the .NET Standard.

This allows to slightly clean up the code by leveraging newer syntactic features.

(best to review ignoring whitespace)

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

How complicated do you think these changes are? Grade on a scale from 1 to 5,
where 1 is a trivial change, and 5 is a deep-reaching and complex change.

This complexity rating applies not only to the complexity apparent in the diff,
but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways.

Testing

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so that you're confident that all the changes work as expected!

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[kazimuth]

I am happy with this. From my reading there are no semantic changes, just formatting, right? Looks nice.

[RReverser]

Yeah all changes are syntax upgrades suggested by IDE once I updated the lang version. Should be a non-functional change.