fix: switch to polygonscan for amoy

[Reinis-FRP]

Motivation

At the time when Amoy testnet contracts were deployed, Polygon listed OKLINK explorer as the primary block explorer and we used it for UMA protocol contract verification. Now that Polygon has switched their primary Amoy testnet to Polygonscan we expect most users would be interacting with UMA contracts through it.

Summary

Switch Amoy testnet block explorer to Polygonscan.

Details

We also now have verified deployed UMA protocol contracts on https://amoy.polygonscan.com/

Testing

Check a box to describe how you tested these changes and list the steps for reviewers to test.

• [ ] Ran end-to-end test, running the code as in production
• [ ] New unit tests created
• [ ] Existing tests adequate, no new tests required
• [ ] All existing tests pass
• [ ] Untested

Issue(s)

Fixes #XXXX