vtls/rustls: support strong CSRNG data

Now that the curl rustls vtls backend is using rustls 0.14 we can
address the weak random situation by using
`rustls_default_crypto_provider_random()` to provide a `Curl_ssl`
`random` callback that fills the provided buffer with cryptographically
secure random data.

The mentions in `docs/` about weak RNG when using rustls are removed as
they are no longer applicable.

Closes #14889

lib/rand.h

@@ -36,11 +36,6 @@ CURLcode Curl_rand_bytes(struct Curl_easy *data,
 #define Curl_rand(a,b,c)   Curl_rand_bytes((a), (b), (c))
 #endif
 
-/* ---- non-cryptographic version following ---- */
-CURLcode Curl_weak_random(struct Curl_easy *data,
-                          unsigned char *rnd,
-                          size_t length);
-
 /*
  * Curl_rand_hex() fills the 'rnd' buffer with a given 'num' size with random
  * hexadecimal digits PLUS a null-terminating byte. It must be an odd number