Bump nim-eth minimum to 0.7.0 (#203)

`TxEip7873` requires nim-eth to be at least 0.7.0.

```
./nim-web3/web3/transaction_signing.nim(98, 32) Error: undeclared identifier: 'TxEip7873'
```

Author: etan-status

web3.nimble

@@ -18,7 +18,7 @@ requires "nim >= 2.0.0"
 requires "chronicles"
 requires "chronos"
 requires "bearssl"
-requires "eth"
+requires "eth >= 0.7.0"
 requires "faststreams"
 requires "json_rpc"
 requires "json_serialization"