[5.4] Change user_id to unsigned integer (#18557)

In the session database table, user_id should be an unsigned integer.

src/Illuminate/Session/Console/stubs/database.stub

@@ -15,7 +15,7 @@ class CreateSessionsTable extends Migration
     {
         Schema::create('sessions', function (Blueprint $table) {
             $table->string('id')->unique();
-            $table->integer('user_id')->nullable();
+            $table->unsignedInteger('user_id')->nullable();
             $table->string('ip_address', 45)->nullable();
             $table->text('user_agent')->nullable();
             $table->text('payload');