Commented out MySQL requirement with explanation (GoogleCloudPlatform#2202)

* Commented out MySQL requirement with explanation

* Address security warning with older Django version

* Updated Django version due to vulnerability

* Address older Django vulnerability

* Update requirements.txt

Author: engelke

appengine/flexible/django_cloudsql/requirements.txt

@@ -1,4 +1,4 @@
-Django==2.1.5
+Django==2.1.9
 mysqlclient==1.4.1
 wheel==0.32.3
 gunicorn==19.9.0

appengine/flexible/hello_world_django/requirements.txt

@@ -1,2 +1,2 @@
-Django==2.1.5
+Django==2.1.9
 gunicorn==19.9.0

appengine/standard_python37/django/requirements.txt

@@ -1,2 +1,2 @@
-Django==2.1.5
+Django==2.1.9
 PyMySQL==0.9.3

container_engine/django_tutorial/requirements.txt

@@ -1,5 +1,7 @@
-Django==2.1.5
-mysqlclient==1.4.1
+Django==2.1.9
+# Uncomment the mysqlclient requirement if you are using MySQL rather than
+# PostgreSQL. You must also have a MySQL client installed in that case.
+#mysqlclient==1.4.1
 wheel==0.32.3
 gunicorn==19.9.0
 psycopg2==2.7.7