scripted-diff: Rename DEFAULT_MAX_MEMPOOL_SIZE to indicate SI unit

Better to be explicit when it comes to sizes to avoid unintentional
bugs. We use MB and KB all over the place.

-BEGIN VERIFY SCRIPT-
find_regex="DEFAULT_MAX_MEMPOOL_SIZE" \
    && git grep -l -E "$find_regex" \
        | xargs sed -i -E "s@$find_regex@\0_MB@g"
-END VERIFY SCRIPT-

src/node/interfaces.cpp

@@ -685,7 +685,7 @@ public:
     CFeeRate mempoolMinFee() override
     {
         if (!m_node.mempool) return {};
-        return m_node.mempool->GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+        return m_node.mempool->GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE_MB) * 1000000);
     }
     CFeeRate relayMinFee() override { return ::minRelayTxFee; }
     CFeeRate relayIncrementalFee() override { return ::incrementalRelayFee; }