Update web server TLS versions for 2.5.0
Now that it's 2019 and we have OpenSSL 1.1.x we can play a bit of musical TLS versions for nginx.
Captive Portal currently has: "TLSv1 TLSv1.1 TLSv1.2" with v1 only remaining for compatibility.
The GUI currently uses: "TLSv1.1 TLSv1.2"
In both cases we can remove the lowest item and add TLSv1.3, to get:
CP: TLSv1.1 TLSv1.2 TLSv1.3
GUI: TLSv1.2 TLSv1.3
Testing locally this works fine for me, so it's good enough to go into 2.5.0 for further testing.