Inform users and prevent PMM server downgrade

Downgrades of PMM are not supported and in practice they can cause some issues:

"
Downgrades are not supported. To go back to using a previous version you must have created a backup of it before upgrading.
"
https://docs.percona.com/percona-monitoring-and-management/setting-up/server/docker.html#upgrade

It would be very nice if PMM could recognize it's downgraded and let the user know somehow. It's very easy to make that mistake, especially when using docker.

 Solutioning options

• bashscript

• pmm init script

Expected result

• CLI warning msg and server UI won't start

• introduce new env var to skip PMM_ALLOW_FORCE_DOWNGRADE to allow downgrade