rubocop: Remove `_` from allowed `Naming/MethodParameterName`
rubocop: Remove `_` from allowed `Naming/MethodParameterName`

- As of RuboCop v0.67.0 (quite some time ago!), this rule allows `_` by
  default as it commonly denotes "this argument is unused". This is not
  set upstream in `AllowedNames`, the code
  [ignores underscores](https://github.com/rubocop/rubocop/commit/4bc7d8c1bc14109525ec370cd5b338a9470bed84#diff-06572a96a58dc510037d5efa622f9bec8519bc1beab13c9f251e97e657a9d4ed)
  entirely.

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