common: Disallow downgrading contracts

`PrevVersion` marks suitable version that we can upgrade from.
However, we can have multiple minor versions, so, currently an upgrade
from v0.19.3 to v0.19.1 is possible. Prevent this with an additional
check.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

common/version.go

@@ -32,7 +32,7 @@ func CheckVersion(from int) {
 	if from < PrevVersion {
 		panic(ErrVersionMismatch + ": expected >=" + std.Itoa(PrevVersion, 10))
 	}
-	if from == Version {
+	if Version <= from {
 		panic(ErrAlreadyUpdated + ": " + std.Itoa(Version, 10))
 	}
 }