Bring back `Devise.activerecord51? and deprecate it

Even though this is considered an internal / non-public / nodoc method,
it seems some libraries relied on it internally, causing some breakage.

Known libraries so far are `devise-security` and
`devise-pwned_password`.

Closes #5580

CHANGELOG.md

@@ -1,6 +1,7 @@
 ### Unreleased
 
-
+* deprecations
+  * Bring back `Devise.activerecord51?` and deprecate it, in order to avoid breakage with some libraries that apparently relied on it.
 
 ### 4.9.1 - 2023-03-31
 

lib/devise.rb

@@ -520,6 +520,14 @@ module Devise
     b.each_byte { |byte| res |= byte ^ l.shift }
     res == 0
   end
+
+  def self.activerecord51? # :nodoc:
+    ActiveSupport::Deprecation.warn <<-DEPRECATION.strip_heredoc
+      [Devise] `Devise.activerecord51?` is deprecated and will be removed in the next major version.
+      It is a non-public method that's no longer used internally, but that other libraries have been relying on.
+    DEPRECATION
+    defined?(ActiveRecord) && ActiveRecord.gem_version >= Gem::Version.new("5.1.x")
+  end
 end
 
 require 'warden'

test/devise_test.rb

@@ -106,4 +106,8 @@ class DeviseTest < ActiveSupport::TestCase
       assert_no_match Devise.email_regexp, email
     end
   end
+
+  test 'Devise.activerecord51? deprecation' do
+    assert_deprecated { Devise.activerecord51? }
+  end
 end