[rails6][UniqueWithinRegionValidator] Use .default_scoped
[rails6][UniqueWithinRegionValidator] Use .default_scoped

Adds a fix for the following deprecation error:

  DEPRECATION WARNING: Class level methods will no longer inherit
  scoping from `create!` in Rails 6.1. To continue using the scoped
  relation, pass it into the block directly. To instead access the full
  set of models, as Rails 6.1 will, use `MiqGroup.default_scoped`.

  (called from validate_each at /Users/nicklamuro/code/redhat/manageiq/lib/unique_within_region_validator.rb:25)

This was added to fix "leaking scopes" in Rails 6.1, but was added as a
deprecation warning first:

- https://github.com/rails/rails/pull/35280
- https://github.com/rails/rails/pull/37727

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