Please provide a `net461` or `net472` project target to match `netstandard2.0` preprocessor

[hotchkj]

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We recently hit a surprising bit of behaviour by MSBuild related to this project. It turns out that the order of framework precedence means that any NET Framework compile ends up pulling the net45 target, despite netstandard2.0 being later, offering more features, less bugs, etc.

In particular this means any net45 specific behaviour that turns out to be a problem simply due to age of the associated language feature, e.g. AsyncLocalManager's remoting, is very hard to avoid.

It would be ideal if the preprocessor usage in the tracer assembly used the NET Standard 2.0 approach (wherever that is necessary) also for a later netxxx target (which will avoid surprises like this).

In theory net461 matches netstandard2.0 in API surface area, but given the Microsoft fine print:

there are several issues with consuming .NET Standard libraries that were built for those versions from .NET Framework 4.6.1 projects. For .NET Framework projects that need to use such libraries, we recommend that you upgrade the project to target .NET Framework 4.7.2 or higher.

This could just be net472 for safety's sake.

[cwe1ss]

I will add this if there's going to be a new release - see #128. There's not much going on here anymore as most work is done in the newer OpenTelemetry project.