node: use uint256::FromUserHex for -minimumchainwork parsing

Removes dependency on unsafe and deprecated uint256S.

This makes parsing more strict, by returning an error
when the input contains more than 64 hex digits.

test/functional/feature_minchainwork.py

@@ -110,7 +110,7 @@ class MinimumChainWorkTest(BitcoinTestFramework):
         self.stop_node(0)
         self.nodes[0].assert_start_raises_init_error(
             ["-minimumchainwork=test"],
-            expected_msg='Error: Invalid non-hex (test) minimum chain work value specified',
+            expected_msg='Error: Invalid minimum work specified (test), must be up to 64 hex digits',
         )