Remove and/or replace support for time based upgrade

[blewater]

Ref Cosmos #8538 consensus failure when upgrading by time.

Describe the solution you'd like
Before the next release, remove the default time-based upgrade mechanism.

Describe alternatives you've considered

• Existing genesis-based
• Opening an issue at the Tendermint team to make them aware of our dynamic block timings.
• A custom or hybrid time scheduling scheme i.e. time + height offset e.g. 22:00 UTC + 10 height.

Additional context
If we end up implementing our own scheme, we should be aware of the chain panic upon setting the time value.

cosmos.upgrade.v1beta1.Plan#time is deprecated, because the SDK stops supporting time-based upgrades in favor or height-based upgrades. If an upgrade Plan is created with a non-empty time, the node will error.
Ref https://github.com/cosmos/cosmos-sdk/releases/tag/v0.43.0