Meld the migration guide for the removal of
MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES into the migration guide for
the strengthening of TLS and X.509 defaults, which is more general. The
information in the SHA-1 section was largely already present in the
strengthening section. It is now less straightforward to figure out how to
enable SHA-1 in certificates, but that's a good thing, since no one should
still be doing this in 2021.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>