Clear process to update `MinimumUntrustedScore`

[kianenigma]

We deployed this last year, and it is a great security addition, but we need a better way to update it.

Currently I mostly do it manually, every quarter or so, through a proposal to the governance.

We need a new system where this value is updated automatically, either via bots offchain, or preferably via a simple onchain mechanism.

The onchain mechanism should still be overwritten by any governance controlled origin.

This is the blocker for https://github.com/paritytech/substrate/pull/11703 and alike as well.