snap: migrate to `base: core22`

This migrates the snap to the new core22 base, aligning with the Mir stack.

It's recommended to create a 22 track for this and continue to security-maintain a 20 track so that users on Ubuntu Core 20 don't have to bring the whole new stack onto their devices.

Ref. https://discourse.ubuntu.com/t/psa-retiring-the-latest-tracks-for-ubuntu-frame-graphics-and-mir-test-tools-action-required/37531