upgrade to polkadot-v1.6

[brenzi]

noteworthy for us

• this will enable XCM teleport of TEER to asset hub with: pallet-xcm: add new flexible transfer_assets() call/extrinsic paritytech/polkadot-sdk#2388 (avoiding the temporary hack in trappist, where they use withdrawTeleport pallet
• better storage version init for migrations: Initialise on-chain StorageVersion for pallets added after genesis paritytech/polkadot-sdk#1297
• needs migration of xcmp-queue

needs integritee-network/pallets#250

we skip v1.4. and v1.5 because ORML skipped them as well from 0.6.7 to 0.7.0
we don't go directly to 1.7 system chains will hop from 1.3 to 1.7 but that's WIP as of today

• 1.7 will introduce XCMv4. better to have a working feature-complete setup with XCMv3 as there may be laggard siblings