curl-openssl: fix SRP check for OpenSSL 3.0

curl-openssl: fix SRP check for OpenSSL 3.0

When OpenSSL 3.0 is built with `--api=3.0` and `no-deprecated`, the SRP
functions exist in the library, but are disabled for user code. Check
if they are actually usable instead of only if they exist. Also, check
for the functions actually required for TLS-SRP.

TLS-SRP support is still enabled if OpenSSL is configured with just
`--api=3.0` or with `--api=1.1.1 no-deprecated`.

Closes https://github.com/curl/curl/pull/8394