Rename login_webarena_sites_v2.py to login_webarena_sites.py

Removes version suffix for cleaner naming. The v2 version is now the
primary login script using YAML-based task definitions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: olesho