Fix an incorrect behavior when using AllCops: MigratedSchemaVersion

[koic]

This PR fixes an incorrect behavior when using AllCops: MigratedSchemaVersion.

This suppresses the following warning when MigratedSchemaVersion is defined on the user side:

$ bundle exec rubocop
(snip)

Warning: AllCops does not support MigratedSchemaVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - DocumentationBaseURL
  - DocumentationExtension
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - NewCops
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion
  - ParserEngine
  - SuggestExtensions
  - ActiveSupportExtensionsEnabled
  - StringLiteralsFrozenByDefault

The default value of AllCops: MigratedSchemaVersion in config/default.yml will change from null to the UNIX epoch time, but typically, there should be no migration files older than the UNIX epoch time.

This is a bug fix related to the plugin migration in #1434.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.