Add release-gate.yml workflow for v2 testing

- Checkout from dev branch for v2 code
- Full setup phase for all test suites and integration tests
- Use FROGBOT_V2_PLATFORM_URL and FROGBOT_V2_ACCESS_TOKEN
- Go 1.21 with caching enabled
- Simplified Bitbucket setup (system deps, Git downgrade, no mesh patch yet)
- Testing without mesh disable to see if it's needed

Author: eyalk007