refactor: migrate management SDK usage from go-auth0 v2 to v3

[developerkunal]

🔧 Changes

Migrates the CLI's Management API usage from go-auth0 v2 to v3. v3 keeps v2's client initialization, package layout, and option pattern, so this is effectively an import-path change plus a naming refresh. It lays the groundwork for the upcoming priority-resource commands, which will be built on v3.

• Updated every github.com/auth0/go-auth0/v2 import to github.com/auth0/go-auth0/v3 across the internal/cli, internal/auth0, and internal/display packages.
• Renamed the v2-flavored identifiers to their v3 equivalents: the managementv2 import alias, the APIV2 struct and its AttackProtectionBotDetectionAPIV2 / EventsAPIV2 interfaces, the NewAPIV2 constructor, the apiv2 field on the cli struct, and initializeManagementClientV2.
• Regenerated the phone-notification-template mock (make test-mocks) against the v3 interface.
• Dropped go-auth0/v2 from go.mod; the repo now depends on v1 + v3.

No command surface, flags, or output behavior change. None of the tightened v3 types (connection attribute identifiers, role pagination, phone backoff enum, federated tokensets, session-transfer device-binding enum) are referenced in the CLI, so the migration required no type fixes.

📚 References

Part of the CLI Priority Resource Coverage effort. This is the first PR in a stack that will add Connections, Client Grants, Flows & Forms, and Sessions + Refresh Tokens commands on top of go-auth0 v3.

🔬 Testing

• go build ./... and go vet ./... pass.
• Unit tests pass for the affected packages (internal/cli, internal/auth0, internal/display).
• make lint surfaces only pre-existing issues in files untouched by this change.
• The generated command docs are unchanged, since no command surface was modified.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)