wolfssl: document v5.0.0 (2021-11-01) as minimum required

[vszakats]

The previously documented version was based on version checks made in
the source. In practice though, curl doesn't build with <5.0.0.

Also:

• bump main internal version check.
• drop superfluous internal version checks.

---

• build also requires a wolfSSL built with --enable-opensslextra. Document this? Drop logic dealing with it missing? Fix to build without? → [FUTURE PR]
• drop logic dealing with wolfSSL <5.0.0.

Bumped from 4.8.0 in 2024 with the introduction of CA caching in
80aa519, due to:

lib/vtls/wolfssl.c:776:6: error: call to undeclared function 'wolfSSL_X509_STORE_up_ref'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  776 |   if(wolfSSL_X509_STORE_up_ref(store)) {
      |      ^

Tested with builds --enable-curl --enable-opensslextra.