Migrate utils to use uv

[samuelhwilliams]

Ticket: https://mhclgdigital.atlassian.net/browse/FSPT-73

Change description

As we've done with all of our other apps now, this moves utils over to using utils for local development and testing.

• Replace constraints/* files with uv_resolution = highest/lowest-direct, so that we are testing the upper and lower
  bounds of our direct dependencies.
• Switch tox over to using tox-uv and tox-gh to decide which tox
  envs to use in which job run (based on python version).
• Bump our minimum pyyaml version to 6.0.1, because 6.0.0 doesn't
  compile for our minimum deps on python 3.12
• Flask 2.2 is not compatible with werkzeug 3.1+, so pin that to werkzeug<3.1. (We can possibly drop flask 2.2 support altogether?)

[MarcUsher]

👍