*) mod_md: update to version 2.4.28
     - When the server starts, it looks for new, staged certificates to
       activate. If the staged set of files in 'md/staging/<domain>' is messed
       up, this could prevent further renewals to happen. Now, when the staging
       set is present, but could not be activated due to an error, purge the
       whole directory. [icing]
     - Fix certificate retrieval on ACME renewal to not require a 'Location:'
       header returned by the ACME CA. This was the way it was done in ACME
       before it became an IETF standard. Let's Encrypt still supports this,
       but other CAs do not. [icing]
     - Restore compatibility with OpenSSL < 1.1. [ylavic]