update to polkadot-sdk unstable2507 (#431)

Support for `polkadot-sdk` release `unstable2507`. This release will be
deployed to Kusama and is supposed the first one on Polkadot.

---------

Signed-off-by: xermicus <cyrill@parity.io>
Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/integration/contracts/BaseFee.sol

@@ -26,6 +26,6 @@ pragma solidity ^0.8;
 
 contract BaseFee {
     constructor() payable {
-        assert(block.basefee == 0);
+        assert(block.basefee > 0);
     }
 }