fix(fx-tunnel-relayer): replace mumbay with amoy

[Reinis-FRP]

Motivation

Tests were breaking with deprecation of Mumbay

Summary

Replaces mumbai with amoy in maticPOSClient init function

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