boostrap-new-os.md doc is outdated or obsolete

[mthalman]

The doc at https://github.com/dotnet/source-build/blob/main/Documentation/boostrap-new-os.md looks to be outdated or perhaps entirely obsolete. For example, it refers to a buildbootstrapcli.sh script that doesn't exist anymore. It should be examined to determine whether any of the content is relevant anymore and updated as appropriate.

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[blucybrb14de]

(See above issue, for details.)

As mentioned in #4725, it would be great if I could get some clarification, for building the bootstrap. I am trying to fix dotnet-sdk, support for Gentoo GNU/Linux x86_64, using LibreSSL, as non-offical support for those systems, and it would be really great, if I could provide that that as a fix.

[am11]

@blucybrb14de, if the bootstrap SDK you are using to build SDK is from Microsoft, then you can use an environment variable to override libssl version: CLR_OPENSSL_VERSION_OVERRIDE=<version of libssl> e.g. CLR_OPENSSL_VERSION_OVERRIDE=46 dotnet run was something we were using on Void Linux musl (with libressl). Note: we are renaming CLR_OPENSSL_VERSION_OVERRIDE to DOTNET_OPENSSL_VERSION_OVERRIDE in dotnet/runtime#110259 for .NET 10.

Ideally, the bootstrap SDK should be built on the platform to avoid running into these issues. The process is documented here: https://github.com/dotnet/source-build/blob/main/Documentation/bootstrapping-guidelines.md