fix: upgrade fixed-merkle-tree from 0.6.1 to 0.7.3

Snyk has created this PR to upgrade fixed-merkle-tree from 0.6.1 to 0.7.3.

See this package in npm:
https://www.npmjs.com/package/fixed-merkle-tree

See this project in Snyk:
https://app.snyk.io/org/hyperledger-bot/project/f688eb37-df53-4c2e-8b47-56cb1b5df9c3?utm_source=github&utm_medium=referral&page=upgrade-pr

integration/polkadot/package.json

@@ -38,6 +38,6 @@
     "websnark": "git+https://github.com/tornadocash/websnark.git#4c0af6a8b65aabea3c09f377f63c44e7a58afa6d",
     "snarkjs": "git+https://github.com/tornadocash/snarkjs.git#869181cfaf7526fe8972073d31655493a04326d5",
     "circomlib": "git+https://github.com/tornadocash/circomlib.git#c372f14d324d57339c88451834bf2824e73bbdbc",
-    "fixed-merkle-tree": "^0.6.0"
+    "fixed-merkle-tree": "^0.7.3"
   }
 }