Migrate to `v2` delegate endpoints

[iamacook]

Deadline: 14th June. We should have this done earlier though for clients to migrate to.

Description

The Transaction Service has created v2 endpoints for delegate management. We should accordingly migrate to them, potentially also releasing v2 endpoints is the payloads do not match correctly.

Requirements

• Migrate to GET /v2/delegates
• Migrate to POST /v2/delegates
• Migrate to POST /v2/delegates/{delegate_address}

Additional information

Inform clients whether the migration is a breaking change after.