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).

[timscharfenort8]

Good work

[MikeMcQuaid]

Makes sense, thanks @dduugg!