[nfc][cmake] mention that some builtins are no longer bundled

[ferdymercury]

but require internet connection

[github-actions]

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[dpiparo]

Thanks. When zstd is merged #21597 and zlib is added to the bundle, this will be merged (and maybe topped up with lzma, which is treated the same.
This nice doc will be complemented with something more substantial in the Release Notes and in the repository as well, to give a clear and simple overview of what builtins are used and what is their version.

[ferdymercury]

Thanks for the feedback Danilo :)

btw, builtin_xxhash looks like another candidate for off-loading to LCG, in the context of compression

[silverweed]

btw, builtin_xxhash looks like another candidate for off-loading to LCG, in the context of compression

AFAIK the policy is to keep anything that is strictly required to build minimal ROOT as vendored code and xxhash is one such dependency (required e.g. to build RNTuple)