Improve Downgrade workflow

[fhagemann]

Follow-up of #57:

Increase [compat] of Requires to 1.1.3 to make Downgrade tests pass.
Make sure that the Downgrade tests are run with the correct julia version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.41%. Comparing base (a126210) to head (c3c6eea).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   53.41%   53.41%           
=======================================
  Files          29       29           
  Lines         996      996           
=======================================
  Hits          532      532           
  Misses        464      464           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[oschulz]

Thanks!