switch email to a primary key (#6064)

switching from a normal index here to a primary index works the same except for adding a `UNIQUE` constraint.

The `DatabaseTokenRepository` deletes existing records with an email first, before creating a new one, so this additional constraint will be okay.

https://github.com/laravel/framework/blob/9.x/src/Illuminate/Auth/Passwords/DatabaseTokenRepository.php#L88

database/migrations/2014_10_12_100000_create_password_resets_table.php

@@ -14,7 +14,7 @@
     public function up()
     {
         Schema::create('password_resets', function (Blueprint $table) {
-            $table->string('email')->index();
+            $table->string('email')->primary();
             $table->string('token');
             $table->timestamp('created_at')->nullable();
         });