tidy-up: use `OPENSSL_VERSION_NUMBER`

tidy-up: use `OPENSSL_VERSION_NUMBER`

Uniformly use `OPENSSL_VERSION_NUMBER` to check for OpenSSL version.
Before this patch some places used `OPENSSL_VERSION_MAJOR`.

Also fix `lib/md4.c`, which included `opensslconf.h`, but that doesn't
define any version number in these implementations: BoringSSL, AWS-LC,
LibreSSL, wolfSSL. (Only in mainline OpenSSL/quictls). Switch that to
`opensslv.h`. This wasn't causing a deeper problem because the code is
looking for v3, which is only provided by OpenSSL/quictls as of now.

According to https://github.com/openssl/openssl/issues/17517, the macro
`OPENSSL_VERSION_NUMBER` is safe to use and not deprecated.

Reviewed-by: Marcel Raad
Closes #12298