Added `CERTIFICATE_VERIFY_MAX_LENGTH` constant

Added `CERTIFICATE_VERIFY_MAX_LENGTH` constant

- Set `CERTIFICATE_VERIFY_MAX_LENGTH` to 65539
  (2 bytes for the algorithm identifier + 2 bytes of signature length
   + 65535 bytes of signature)
- Changed `SSL3_RT_MAX_PLAIN_LENGTH` to `CERTIFICATE_VERIFY_MAX_LENGTH`
  in `statem_srvr.c` and `statem_clnt.c`

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20486)