Migrate app-create-verify to pyproject.toml

[gwdio]

• Replaces python -m pip install -Ur ./briefcase-template/requirements.txt with python -m pip install --group ./briefcase-template/pyproject.toml:create-verify
• Updates pip to ensure support of the --group feature
• The new create-verify dependency group in briefcase-template contains exactly black, flake8, and toml — the only packages this workflow directly invokes
• Drops the extraneous briefcase, pre-commit, pytest, and tox that were previously installed but unused by this workflow

Requirements.txt in briefcase-template is to be deprecated. This is the only location that uses it.
Group in pyproject.toml pins exactly the surface this workflow needs.

Fixes #355

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Opus 4.7, Claude Sonnet 4.6

[freakboy3742]

👍