switch from Google recaptcha to cloudflare turnstile

[williamstein]

Why?

We currently support the free "Google Recaptcha Classic" in CoCalc. Google is killing this product in a typically confusing way; it also has possibly negative privacy implications.
Cloudflare has a competing product:

https://www.cloudflare.com/application-services/products/turnstile/

and this is clearly something Cloudflare is also very good at, since they use it for anti DDOS protection, etc. The price is a few dollars per month.

1. Remove the optional Google Recaptcha code entirely from cocalc
2. Add optional Turnstile support to cocalc.