Exclude some stdlib gems from RBI generation

[dduugg]

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I propose excluding stdlib gems whose rbi files are already provided by sorbet: https://github.com/sorbet/sorbet/tree/master/rbi/stdlib

This saves some git bloat, and the sorbet versions are mostly better (they often include stricter type definitions, though AFAIK they are not automatically updated when new gem versions are released).

[dduugg]

Note that this was being marked false anyway, presumably due to conflicts with the sorbet version. (Might be worth finding the diff and upstreaming any fix, if someone feels motivated…)