rubocop: Set inherit_mode for allowed Naming/MethodParameterName

- As I was looking through the RuboCop configs for
  `Naming/MethodParameterName`, since there was a fairly long-standing
  TODO here, I found that in the default RuboCop configs the method
  names `to` and `io` are
  [allowed by default](41a8249b35/config/default.yml (L2794)).
- But we weren't inheriting those allowed names correctly from the
  default configuration. This fixes that so that we get future
  enhancements automatically for this particular cop allowlist.

Library/.rubocop.yml

@@ -371,6 +371,9 @@ Metrics/PerceivedComplexity:
 # allow those that are standard
 # TODO: try to remove some of these
 Naming/MethodParameterName:
+  inherit_mode:
+    merge:
+      - AllowedNames
   AllowedNames:
     - "a"
     - "b"
@@ -383,14 +386,12 @@ Naming/MethodParameterName:
     - "ff"
     - "fn"
     - "id"
-    - "io"
     - "o"
     - "p"
     - "pr"
     - "r"
     - "rb"
     - "s"
-    - "to"
     - "v"
 
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