Fix an incorrect behavior when using AllCops: MigratedSchemaVersion

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

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

```console
$ 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.

Author: koic

changelog/fix_an_incorrect_behavior_when_using_migrated_schema_version.md

@@ -0,0 +1 @@
+* [#1444](https://github.com/rubocop/rubocop-rails/pull/1444): Fix an incorrect behavior when using `AllCops: MigratedSchemaVersion`. ([@koic][])

config/default.yml

@@ -28,7 +28,7 @@ AllCops:
   # By specifying `MigratedSchemaVersion` option, migration files that have been migrated can be ignored.
   # When `MigratedSchemaVersion: '20241231000000'` is set. Migration files lower than or equal to '20250101000000' will be ignored.
   # For example, this is the timestamp in db/migrate/20250101000000_create_articles.rb.
-  MigratedSchemaVersion: ~
+  MigratedSchemaVersion: '19700101000000' # NOTE: Used as a sentinel value for the UNIX epoch time.
 
 Lint/NumberConversion:
   # Add Rails' duration methods to the ignore list for `Lint/NumberConversion`