Remove v1 user-creation path

[michael-richey]

Summary

Stacked on #639. Remove the legacy v1 user-creation implementation now that compatible destination APIs can receive an explicit handle through the v2 path.

Breaking change

--use-v1-user-api and DD_USE_V1_USER_API no longer select a v1 execution path. They remain recognized only so existing command lines, environment settings, and configuration files fail fast with migration guidance instead of silently reverting to default v2 behavior.

Operators should either:

• Switch to --preserve-user-handle / DD_PRESERVE_USER_HANDLE when the destination supports explicit handles during user creation, or
• Pin to an older version if the legacy v1 path is still required.

Changes

• Remove the v1 create branch and its post-create lookup helpers.
• Remove the runtime use_v1_user_api configuration field.
• Keep the old option and environment variable hidden as compatibility guards that raise a usage error when enabled.
• Remove the v1 README section and implementation-specific tests.
• Add regression tests for direct configuration, CLI, and environment-variable failure paths.

Test plan

• Red/green tests confirm legacy settings fail before API or storage work
• sync --help and migrate --help show --preserve-user-handle and hide the removed option
• Full Python 3.11 unit suite passes