Update sBTC web app domain

[Andre Serrano]

The current domain for sBTC app on testnet is https://bridge.stx.eco/start

We should update this to remove bridge and stx from the domain. It could be "mint.sbtc.world" for example.

We previously used sbtc.world for the testnet bridge app, but that has not been updated.

[Mike Cohen]

Note that the app supports testnet and mainnet on the same domain (via a query string parameter).

[Friedger]

Current url is https://bridge.sbtc.tech/?net=testnet