Fix builder submission gate message

[luch91]

Summary

• fix the builder submission gate error message to reference the full Builder journey
• add a regression test for users who have Builder Welcome but do not yet have builder status

Verification

• python3 manage.py test contributions.tests.test_submission_permissions -v 2
• verified in the repo's Python 3.13 Docker environment

[JoaquinBN]

Thanks for the catch, @luch91! This is a valid fix, but it's a small UI-facing string change and not something we're prioritizing right now. We're in the middle of a full submissions review and rebranding — after that we'll be setting up the repo with organized issues for the community to contribute to. Appreciate you taking the time and keep an eye out!