* Fixed PhpDoc in Migration.php (revert #18886)
Fixed the return type of `up()`, `down()`, `safeUp()` and `safeDown()` functions to match the described behavior.
The current implementation checks for `false`, "all other return values mean the migration succeeds".
This PR also reverts #18886 since it was a code fix for what was documentation error.
Allow all possible values for `yii\db\Migration` functions `up()`, `down()`, `safeUp()` and `safeDown()`