node: increase Polygon minimum confirmation to 512

It appears that in spite of exceeding the recommended value by 2x,
256 offers insufficient safety margin given that we saw two reorgs
of depths 178 and 187 recently: https://polygonscan.com/blocks_forked

This will increase confirmation delay from ~10 minutes to ~20 minutes.

commit-id:cf2b1503

node/cmd/guardiand/node.go

@@ -638,10 +638,10 @@ func runNode(cmd *cobra.Command, args []string) {
 				*polygonRPC, polygonContractAddr, "polygon", common.ReadinessPolygonSyncing, vaa.ChainIDPolygon, lockC, nil,
 				// Special case: Polygon can fork like PoW Ethereum, and it's not clear what the safe number of blocks is
 				//
-				// Hardcode the minimum number of confirmations to 256 regardless of what the smart contract specifies to protect
+				// Hardcode the minimum number of confirmations to 512 regardless of what the smart contract specifies to protect
 				// developers from accidentally specifying an unsafe number of confirmations. We can remove this restriction as soon
 				// as specific public guidance exists for Polygon developers.
-				256).Run); err != nil {
+				512).Run); err != nil {
 			return err
 		}
 		if err := supervisor.Run(ctx, "avalanchewatch",