Update "upgrading-from-older-ha-versions"

Update upgrading-from-older-ha-versions:
The ARM contains the following upgrade advice.

Do not send the ha-maintenance-start command while the server running the old 
hook library is still online. The server receiving this command will return an error.

This may need to be updated as all supported Kea versions contain the "ha-maintenance-start" command

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information