httpspush.html: update with latest OpenSSH information
Starting with OpenSSH version 7.8p1 (2018-08-23), the default format
for OpenSSH private keys has become a new, specific-to-OpenSSH format.
In order to be used for https pushing, the private key must be in
a compatible format that can interoperate with the https software.
Since OpenSSH version v8.1p1 (2019-10-09), the PKCS8 format (see
RFC 5958) can be used. Prior to OpenSSH version v8.1p1, the older
PEM format must be used.
Add instructions detailing how to inspect the RSA private key and
determine whether or not it's in a compatible format and, if not,
how to convert it to a compatible format.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>