Add a CHANGES entry for BN_generate_prime_ex

BN_generate_prime_ex no longer avoids factors 3..17863 in p-1
when not computing safe primes.
Reviewed-by: NTomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9387)

CHANGES

@@ -9,7 +9,14 @@
 
  Changes between 1.1.1e and 1.1.1f [xx XXX xxxx]
 
-  *)
+  *) Revised BN_generate_prime_ex to not avoid factors 3..17863 in p-1
+     when primes for RSA keys are computed.
+     Since we previously always generated primes == 2 (mod 3) for RSA keys,
+     the 2-prime and 3-prime RSA modules were easy to distinguish, since
+     N = p*q = 1 (mod 3), but N = p*q*r = 2 (mod 3). Therefore fingerprinting
+     2-prime vs. 3-prime RSA keys was possible by computing N mod 3.
+     This avoids possible fingerprinting of newly generated RSA modules.
+     [Bernd Edlinger]
 
  Changes between 1.1.1d and 1.1.1e [17 Mar 2020]
   *) Properly detect EOF while reading in libssl. Previously if we hit an EOF