Investigate the removal of the V2 API

[ccostino]

With the recent work we've been doing after our investigation of the message sending flow path, one of the things we came across was the presence of the V2 API endpoints, which is currently the only place that contains references to the SIMULATED_NUMBERS in our version of Notify.gov.

We're not sure that we still need these endpoints given the modifications we've made to the codebase and if not, it'd be good to remove these so we're not maintaining code and tests for things we're not using.

Implementation Sketch and Acceptance Criteria

• Investigate the V2 API endpoints and verify that we're not making any use of them
• Compare our use with the UK Notify V2 API to see if there's been work that we've missed that we may want to incorporate into Notify.gov

If it's determined that we no longer need this and there's no benefit to updating or improving it, then remove the code and any associated integration points and tests.

If there is benefit to keeping the code and making future improvements and enhancements to it, then write up a proposal ADR for what we should do in the future.

Security Consideration

• We don't want to maintain code that we don't intend to use and isn't providing any functionality that our users are depending on.