[helm] update default image tag to devnet
[helm] update default image tag to devnet

Original image tag was hard coded to previous release. This makes it
easier to maintain, and also adds an additional check for backwards
compat.

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra