ENV: deprecate old methods.

[MikeMcQuaid]

These should have been deprecated long ago but we didn’t really have the necessary framework to do so.

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[nijikon]

Shouldn't this be removed as well?

[MikeMcQuaid]

Technically a valid API in env :std. I guess there's an argument that we should remove any of these that aren't still used/needed in homebrew/core (e.g. ENV.libxml2 is because we use in in test do which uses env :std). Thoughts?

Still a valid API