h3: add support for ngtcp2 with AWS-LC builds

h3: add support for ngtcp2 with AWS-LC builds

```
curl 8.4.0-DEV (x86_64-apple-darwin) libcurl/8.4.0-DEV (SecureTransport) AWS-LC/1.15.0 nghttp2/1.56.0 ngtcp2/0.19.1 nghttp3/0.15.0
Release-Date: [unreleased]
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS HSTS HTTP2 HTTP3 HTTPS-proxy IPv6 Largefile MultiSSL NTLM SSL threadsafe UnixSockets
```

Also delete an obsolete GnuTLS TODO and update the header comment in
`FindNGTCP2.cmake`.

Reviewed-by: Daniel Stenberg
Closes #12066