4.x upgrade note: Mention issues with GnuTLS 3.7.0
4.x upgrade note: Mention issues with GnuTLS 3.7.0

Duplicate intermediate certificates are known to cause issues with GnuTLS 3.7.0 based clients on servers not based on GnuTLS (e.g. OpenSSL-based servers). Document this in the according upgrade note in README.md.

See https://gitlab.com/gnutls/gnutls/-/issues/1131
diafygi/acme-tiny