Update benchmarks

[idg10]

We're currently using BenchmarkDotNet v0.12.0. On trying to move to 0.12.1, it became apparent that this is a significant change, despite what you might guess from the version numbers. The code no longer compiles with the newer version.

We should make all the necessary changes to adopt the latest version, and we should also ensure that automated collection of benchmark output in the build process continues to work.

It might also be a good time to look at whether to start collecting benchmarks for .NET 5.

[jongeorge1]

Note - have told dependabot to ignore this dependency using @dependabot ignore this dependency. We need to make sure it stops ignoring it once this issue is complete.