Adjust upgradability test for multiple protocol updates

[staffik]

See https://near.zulipchat.com/#narrow/stream/295302-general/topic/Protocol.20version.2070.

We might want to:

• Change PEER_MIN_ALLOWED_PROTOCOL_VERSION to 1 (see comment).
• Adjust the upgradable.py test to correctly execute multiple protocol upgrades and force neard to only ever upgrade by 1 (see comment).