pd: implement no op migration to v0.81.0

[cronokirby]

This implements a migration to version 0.81.0, which doesn't require any logic.

This will break PD, but targets the new release branch.

[conorsch]

Tested locally by creating a devnet, then:

1. staking, bonding
2. proposing upgrade plan
3. voting
4. confirming halt
5. running pd migrate to apply the change: ✔️
6. confirming chain restarts on new version of pd: ❌ did not work

Step 6 failed because of the version mismatch between pd and local state: we must also increment APP_VERSION in the code so that the startup version check introduced in #4919 is satisfied. I pushed a commit to do just that.