Disable Active Record partial_inserts by default in Rails 7.0

Ref: rails#42355

The justification for `partial_inserts` back in 2012
(144e869) was:

> This is more efficient, and also means that it will be safe to remove
> database columns without getting subsequent errors in running app processes
> (so long as the code in those processes doesn't contain any references to the
> removed column).

But since then `ignored_columns` is a much more reliable way to safely remove a
column, and I doubt the reduced query size really help much.

Additionally, `partial_inserts` prevent removing the default value of a column
in a safe way.

Author: byroot

activerecord/CHANGELOG.md

@@ -1,11 +1,27 @@
+*   `partial_inserts` is now disabled by default in new apps.
+
+    This will be the default for new apps in Rails 7. To opt in:
+
+    ```ruby
+    config.active_record.partial_inserts = false
+    ```
+
+    If a migration remove the default value of a column, this option
+    would cause old processes to no longer be able to create new records.
+
+    If you need to remove a column, you should first use `ignored_columns`
+    to stop using it.
+
+    *Jean Boussier*
+
 *   Rails can now verify foreign keys after loading fixtures in tests.
 
     This will be the default for new apps in Rails 7. To opt in:
-    
+
     ```ruby
     config.active_record.verify_foreign_keys_for_fixtures = true
     ```
-    
+
     Tests will not run if there is a foreign key constraint violation in your fixture data.
 
     The feature is supported by SQLite and PostgreSQL, other adapters can also add support for it.

railties/lib/rails/application/configuration.rb

@@ -229,6 +229,7 @@ def load_defaults(target_version)
 
           if respond_to?(:active_record)
             active_record.verify_foreign_keys_for_fixtures = true
+            active_record.partial_inserts = false
           end
         else
           raise "Unknown version #{target_version.to_s.inspect}"