Update versioning scheme for Microsoft.DotNet.Cecil

[sbomer]

Currently we build Microsoft.DotNet.Cecil (from dotnet/cecil) using a versioning scheme that matches upstream (jbevain/cecil). This is left over from when we used to depend on the upstream Mono.Cecil in dotnet/linker (see https://github.com/dotnet/linker/pull/1515/files for more context).

Now that we no longer depend on the upstream build, we could adopt the common versioning scheme used by other components.

[dotnet-policy-service]

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