[On Hold] Upgrade to V3 Signature

[JLKwong]

Problem: Treehouses Remote is currently using v1 signature, and Flutter builds v2, but the highest signature is v3 (v4 is Coming Soon™)

Research:
No official Flutter documentation, but found this Android documentation:
https://source.android.com/security/apksigning/v3

The difference with v3 is that it can now switch certificates.

No known Flutter command to use v3 signature.

Status:
Pending method to sign with v3 instead of v1/v2.