Use dolfinx v0.7.0 in CI

[jhdark]

Proposed changes

fix for issue #600

With this change, all the tests on the CI will be done using dolfinx v0.7.0 which we are using locally, can be updated once we we to use a newer updated version

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3227dd3) 98.44% compared to head (732f84e) 98.44%.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx     #658   +/-   ##
========================================
  Coverage    98.44%   98.44%           
========================================
  Files           23       23           
  Lines          964      964           
========================================
  Hits           949      949           
  Misses          15       15           

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