added langversion to benchmark csproj

[JoshLozensky]

Upping the benchmark language version to 12. Benchmarks didn't seem to see any significant difference to me it looks like noise rather than any real difference in the benchmarks.

"diff in mean" = C#v12 - C#v10 .... so negative numbers mean the upgrade ran faster and vice versa

Test Name	diff in mean (μs)	margin of error(μs)	outside margin of error
JsonWebTokenHandler_CreateJWE	2.9	1.62	yes
JsonWebTokenHandler_CreateToken	-35.5	9.46	yes
JsonWebTokenHandler_ValidateTokenAsync	0.13	0.084	yes
JwtSecurityTokenHandler_CreateJWE	5.5	1.49	yes
JwtSecurityTokenHandler_ValidateTokenAsync	0.34	0.142	yes
ReadJWE_FromString	-0.233	0.0905	yes
SignSpanWithArrayPoolRSA	3.3	2.07	yes
JsonWebTokenHandler_ValidateJWEAsync	-0.05	0.854	no
JwtSecurityTokenHandler_ValidateJWEAsync	0.2	0.347	no
ReadJWE_FromMemory	-0.037	0.0755	no
ReadJWS_FromMemory	0.034	0.0365	no
ReadJWS_FromString	0.034	0.0821	no
SHRHandler_CreateSignedHttpRequest	11	20.7	no
SHRHandler_ValidateSignedHttpRequestAsync	0.25	0.343	no
SignDotnetCreatingBufferRSA	0.6	1.65	no
SignSpanWithFixedBufferRSA	-2.3	4.48	no

[keegan-caruso]

what was the environment used?

[jennyf19]