Fix benchmarks for netcoreapp5.0

[chrisdcmoore]

When trying to run the benchmarks with Microsoft.NETCore.App 5.0.0-preview.5.20278.1, they would fail with the following stacktrace

Unhandled exception. System.NotSupportedException: Unknown .NET Framework
   at BenchmarkDotNet.Portability.RuntimeInformation.GetCurrentRuntime()
   at BenchmarkDotNet.Characteristics.Resolver.<>c__DisplayClass1_0`1.<Register>b__0(CharacteristicObject obj)
   at BenchmarkDotNet.Characteristics.Resolver.Resolve[T](CharacteristicObject obj, Characteristic`1 characteristic)
   at BenchmarkDotNet.Characteristics.CompositeResolver.Resolve[T](CharacteristicObject obj, Characteristic`1 characteristic)
   at BenchmarkDotNet.Characteristics.CharacteristicObject.ResolveValue[T](Characteristic`1 characteristic, IResolver resolver)
   at BenchmarkDotNet.Toolchains.ToolchainExtensions.GetToolchain(Job job)
   at BenchmarkDotNet.Running.BenchmarkRunnerClean.<>c__DisplayClass12_0.<GetSupportedBenchmarks>b__2(BenchmarkCase benchmark)
   at System.Linq.Enumerable.WhereArrayIterator`1.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at BenchmarkDotNet.Running.BenchmarkRunnerClean.<>c__DisplayClass12_0.<GetSupportedBenchmarks>b__0(BenchmarkRunInfo info)
   at System.Linq.Enumerable.SelectArrayIterator`2.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at BenchmarkDotNet.Running.BenchmarkRunnerClean.GetSupportedBenchmarks(BenchmarkRunInfo[] benchmarkRunInfos, ILogger logger, IResolver resolver)
   at BenchmarkDotNet.Running.BenchmarkRunnerClean.Run(BenchmarkRunInfo[] benchmarkRunInfos)
   at BenchmarkDotNet.Running.BenchmarkSwitcher.RunWithDirtyAssemblyResolveHelper(String[] args, IConfig config)
   at BenchmarkDotNet.Running.BenchmarkSwitcher.Run(String[] args, IConfig config)
   at Npgsql.Benchmarks.Program.Main(String[] args) in C:\Users\Chris\src\npgsql\test\Npgsql.Benchmarks\Program.cs:line 8

This is due to the verison of BenchmarkDotNet being used not supporting the .NET Core 5 -> .NET 5 rebrand (see dotnet/BenchmarkDotNet#1399)

This PR:

• bumps BenchmarkDotNet to the latest stable version at time of writing (0.12.1) which includes support for runtime detection post-rebrand
• fixes up the build post-package-upgrade by replacing usages of now-obsolete methods with their new equivalents

The benchmarks can now be run on post-rebrand .NET 5 runtimes.

[YohDeadfall]

Thank you!