Fix Rails cop enablement now EnabledByDefault is set

- These were `Enabled: false` for the whole group of Rails cops, so my
  removing the `Enabled: true` cops (what I thought was "redundant"
  config in the previous commit) meant the specific ones we wanted to
  use weren't enabled at all.
- Instead, let's reverse that by unsetting `Enabled: false` for the
  whole group, and then disabling the specific cops with offenses that
  we haven't got around to deciding if we care about yet.

Library/.rubocop.yml

@@ -235,8 +235,6 @@ Performance/OpenStruct:
   Enabled: false
 
 Rails:
-  # Selectively enable what we want.
-  Enabled: false
   # Do not use ActiveSupport in RuboCops.
   Exclude:
     - "Homebrew/rubocops/**/*"
@@ -244,6 +242,22 @@ Rails:
 Rails/Delegate:
   Enabled: false # TODO
 
+# TODO: This group of cops comes from `EnabledByDefault: true`. We should maybe enable the ones with < 100 offenses.
+Rails/ArelStar:
+  Enabled: false
+Rails/Date:
+  Enabled: false
+Rails/RakeEnvironment:
+  Enabled: false
+Rails/Pluck:
+  Enabled: false
+Rails/SaveBang:
+  Enabled: false
+Rails/SkipsModelValidations:
+  Enabled: false
+Rails/TimeZone:
+  Enabled: false
+
 # Intentionally disabled as it doesn't fit with our code style.
 RSpec/AnyInstance:
   Enabled: false