chore: upgrade to polkadot 0.9.16

[sander2]

• [BREAKING] added preimage pallet.The scheduler pallet changed, forcing a change in our code. We have two options: either we add the new preimage pallet or we use the default implementation. In the latter case, behavior would be unchanged. However, I went with the first option because it gives greater flexability: the current behavior is unchanged, but it makes it possible to schedule a call (e.g. runtime upgrade) without immediately uploading it. Rather, you would give a hash of the call, and upload it later. This allows democracy to be more efficient in the future because you don't need to upload a new runtime just to make the proposal. This will still require a change in our democracy pallet, but it will not be a breaking change for our clients.
• Added migration for the scheduler pallet (better safe than sorry)
• Updated handling of xcm paths: they may now get canonicalized and we need to be able to handle both the old and the new representation. Preimage registrar and Scheduler integration paritytech/substrate#10356
• Pallets now need all storage items to implement MaxEncodedLen by default. I tried to implement this where possible, but I ran into an problem where fixedpoint types do not implement it. I opened an issue on the substrate repo for this. Also, our usage of sets and vecs would complicate implementing it anyway. In the future we might want to see if we can find alternatives to the vec usage in e.g. the vault struct. In the end I resorted to opting out of the requirement by using #[pallet::without_storage_info] for most pallets.
• Upgraded compiler version. I was getting cargo errors complaining that named-profiles was not yet stabilized.
• Lots of smaller changes

[gregdhill]

Notes:

• AccountId no longer implements Default
• Runtime version now includes state_version