Do not stub raw connection internals

A flaky test appeared because of this stubbing.

lib/online_migrations/change_column_type_helpers.rb

@@ -118,7 +118,7 @@ def initialize_columns_type_change(table_name, columns_and_types, **options)
           type_cast_functions[column_name] = type_cast_function if type_cast_function
           tmp_column_name = conversions[column_name]
 
-          if raw_connection.server_version >= 11_00_00
+          if database_version >= 11_00_00
             if primary_key(table_name) == column_name.to_s && old_col.type == :integer
               # For PG < 11 and Primary Key conversions, setting a column as the PK
               # converts even check constraints to NOT NULL column constraints
@@ -497,7 +497,7 @@ def __copy_exclusion_constraints(table_name, from_column, to_column)
       def __set_not_null(table_name, column_name)
         # For PG >= 12 we can "promote" CHECK constraint to NOT NULL constraint:
         # https://github.com/postgres/postgres/commit/bbb96c3704c041d139181c6601e5bc770e045d26
-        if raw_connection.server_version >= 12_00_00
+        if database_version >= 12_00_00
           execute(<<~SQL)
             ALTER TABLE #{quote_table_name(table_name)}
             ALTER #{quote_column_name(column_name)}

lib/online_migrations/schema_statements.rb

@@ -439,7 +439,7 @@ def swap_column_names(table_name, column1, column2)
     def add_column_with_default(table_name, column_name, type, **options)
       default = options.fetch(:default)
 
-      if raw_connection.server_version >= 11_00_00 && !Utils.volatile_default?(self, type, default)
+      if database_version >= 11_00_00 && !Utils.volatile_default?(self, type, default)
         add_column(table_name, column_name, type, **options)
       else
         __ensure_not_in_transaction!
@@ -465,7 +465,7 @@ def add_column_with_default(table_name, column_name, type, **options)
           add_not_null_constraint(table_name, column_name, validate: false)
           validate_not_null_constraint(table_name, column_name)
 
-          if raw_connection.server_version >= 12_00_00
+          if database_version >= 12_00_00
             # In PostgreSQL 12+ it is safe to "promote" a CHECK constraint to `NOT NULL` for the column
             change_column_null(table_name, column_name, false)
             remove_not_null_constraint(table_name, column_name)

test/support/minitest_helpers.rb

@@ -121,8 +121,8 @@ def with_partial_writes(value, &block)
   end
 
   def with_postgres(major_version, &block)
-    pg_connection = ActiveRecord::Base.connection.raw_connection
-    pg_connection.stub(:server_version, major_version * 1_00_00, &block)
+    connection = ActiveRecord::Base.connection
+    connection.stub(:database_version, major_version * 1_00_00, &block)
   end
 
   def ar_version