`ethexe`: improve announce from block identification in RPC (not expired announce)

[grishasobol]

// TODO: not perfect solution, better to take the last synced block, and iterate back until

In file ethexe/rpc/src/utils.rs

gear/ethexe/rpc/src/utils.rs

Lines 65 to 67 in 29808df

	}
	/// NOTE: does not return latest computed announce - instead use announce from latest prepared block.