Update Ubuntu version on GitHub action

[aptiko]

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

[codecov[bot]]

Codecov Report

Merging #497 (e183554) into master (e6d7509) will decrease coverage by 0.06%. The diff coverage is 98.41%.

:exclamation: Current head e183554 differs from pull request most recent head ff39849. Consider uploading reports for the commit ff39849 to get more accurate results

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
- Coverage   98.30%   98.25%   -0.06%     
==========================================
  Files          41       41              
  Lines        2127     2115      -12     
==========================================
- Hits         2091     2078      -13     
- Misses         36       37       +1     

Impacted Files	Coverage Δ
enhydris/admin/__init__.py	96.42% <ø> (-0.35%)	:arrow_down:
enhydris/api/csv.py	100.00% <ø> (ø)
enhydris/api/permissions.py	100.00% <ø> (+5.00%)	:arrow_up:
enhydris/api/serializers.py	95.60% <ø> (-0.19%)	:arrow_down:
enhydris/api/urls.py	100.00% <ø> (ø)
enhydris/models/timeseries_group.py	93.40% <ø> (-1.80%)	:arrow_down:
enhydris/telemetry/forms.py	100.00% <ø> (ø)
enhydris/telemetry/views.py	98.31% <ø> (ø)
enhydris/api/views.py	97.89% <96.00%> (-0.37%)	:arrow_down:
enhydris/models/timeseries.py	98.52% <98.36%> (-0.44%)	:arrow_down:
... and 7 more

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