Remove SuppressTfmSupportBuildWarnings usage

[MichaelSimons]

SuppressTfmBuildWarnings=true was added as an environment variable in dotnet/installer#15151 to workaround an aspnetcore issue related to not all repos being moved to a net8.0 tfm. That setting is quite dangerous and not intended to be used in our stack. It should be removed as soon as possible.

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