Tooling updates (Python, Django, Wagtail versions), and coverage summary

[zerolab]

• Bumps min Python version to 3.11 (both bedrock and kitsune are on it)
• Adds Django 5.1 and Wagtail 6.2 to the test matrix
• adjusts the GitHub Actions accordingly and
  • Only runs one test combination for sqlite (fewer machine cycles wasted)
  • Bumps Postgres to 13 as 12 is soon out of support (https://endoflife.date/postgresql)
• Makes tox and pytest a little less verbose in CI
• Outputs the coverage report in the actions summary. For example: https://github.com/mozilla/wagtail-localize-smartling/actions/runs/10322014048#summary-28576532717
• Uses COVERAGE_CORE=sysmon in Python 3.12. This brings the test time from ~6m28s to ~4m42s which is now closer to the Python 3.11 timings
• Runs pyupgrade with a 3.11+ target (git ls-files --others --cached --exclude-standard -- '*.py' | xargs pyupgrade --py311-plus)

[zerolab]

@stevejalim I've addressed the comments in 55e9661 . Thank you for the feedback