dotnet: use openssl@3

[alebcay]

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So far, this seems to work on my local machine. We'll see how CI fares. Upstream notes that dotnet 7.x should support OpenSSL 3.

This PR does not un-deprecate dependents. Open to trying to un-deprecate everything in one try, but I think doing it separately hopefully allows for faster feedback from CI.

[alebcay]

The 13-arm64 failure is fixed in dotnet/runtime@b014ab8, and backported to 7.0.107.

source-build no longer supports macOS after ~7.0.105 (or, perhaps proper support has already been absent and we have been kludging something together to get us along to this point, but keeping up is proving to be onerous); macOS source-build support may return in .NET 9 - dotnet/source-build#2909.