Change static this to class reference (#13510)

Co-authored-by: Sascha Depold <sdepold@users.noreply.github.com>

lib/sequelize.js

@@ -1121,7 +1121,7 @@ class Sequelize {
     if (!ns || typeof ns !== 'object' || typeof ns.bind !== 'function' || typeof ns.run !== 'function') throw new Error('Must provide CLS namespace');
 
     // save namespace as `Sequelize._cls`
-    this._cls = ns;
+    Sequelize._cls = ns;
 
     // return Sequelize for chaining
     return this;