docs: add callGasLimit default change to breaking change list in UPGRADE-GUIDE

UPGRADE-GUIDE.md

@@ -431,6 +431,7 @@ sending account to cover the maximum cost.
 - default `coinbase` (`eth_coinbase` RPC call) is now the `0x0` address (fixes #201)
 - `sender doesn't have enough funds to send tx` errors are now prefixed with `VM Exception while processing transaction`
 - `logs` subscription events are emitted before `newHeads` events
+- the default `callGasLimit` has changed from `Number.MAX_SAFE_INTEGER` ($2^{53} - 1$) to `50_000_000`.
 
 
 
@@ -445,4 +446,4 @@ very rare in testing scenarios, but should be something you are aware of. We'll
 be addressing this shortcoming in a future EIP and release by extending eth_call
 with an option to run the transaction at a certain _index_ in the specified
 block (you likely won't ever see this option enabled on public nodes, like
-Infura, as it can be a very CPU-intensive process).
+Infura, as it can be a very CPU-intensive process).