[openssl/openssl] 174d16: Add empty migration guide for 3.1

[h-vetinari]

  Branch: refs/heads/master
  Home:   https://github.com/openssl/openssl
  Commit: 174d166bc3e5e27d4aef08a5fa4e932e22e8f3a7
      https://github.com/openssl/openssl/commit/174d166bc3e5e27d4aef08a5fa4e932e22e8f3a7
  Author: H. Vetinari <h.vetinari at gmx.com>
  Date:   2023-01-11 (Wed, 11 Jan 2023)

  Changed paths:
    M doc/man7/migration_guide.pod

  Log Message:
  -----------
  Add empty migration guide for 3.1

Fixes #19953

CLA: trivial

Reviewed-by: Paul Dale <pauli at openssl.org>
Reviewed-by: Matt Caswell <matt at openssl.org>
Reviewed-by: Tomas Mraz <tomas at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20008)