[6.0.1xx] Remove System.Security.Cryptography 4.7.0 from SBRP

[ellahathaway]

The Stage 2 tarball build failed due to msbuild 17.0.1's dependency on System.Security.Cryptography.4.7.0, which was removed from 6.0 in dotnet/source-build-reference-packages#939.

This wasn't an issue in Stage 1 as the required package was in the previously-source-built artifacts. To fix the Stage 2 build, System.Security.Cryptography.4.7.0 was added back to SBRP temporarily. Since msbuild fixed its dependency on this package in dotnet/msbuild#9867, we can remove System.Security.Cryptography.4.7.0 from SBRP once the next release occurs and the previously-source-built artifacts have been updated.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichaelSimons]

[triage] This can be removed once we rebootstrap after the May servicing release.