feature: Drop Python 3.7 support, Django 2.2, 3.0 and 3.1 support

[fsbraun]

Description

This PR prepares django-filer for it's 3.0 release. It drops Python 3.7 support and Django 2.2, 3.0, 3.1 support.

Github actions are updates and so is tox.ini.

Related resources

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Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[codecov[bot]]

Codecov Report

Merging #1363 (47dc19c) into master (43434f7) will decrease coverage by 0.14%. The diff coverage is 72.09%.

@@            Coverage Diff             @@
##           master    #1363      +/-   ##
==========================================
- Coverage   73.29%   73.15%   -0.14%     
==========================================
  Files          73       73              
  Lines        3325     3323       -2     
  Branches      541      540       -1     
==========================================
- Hits         2437     2431       -6     
- Misses        717      720       +3     
- Partials      171      172       +1     

Impacted Files	Coverage Δ
filer/__init__.py	100.00% <ø> (ø)
filer/admin/permissionadmin.py	88.23% <0.00%> (ø)
filer/admin/views.py	65.71% <ø> (ø)
filer/fields/file.py	64.44% <ø> (ø)
filer/fields/folder.py	40.00% <ø> (ø)
filer/models/clipboardmodels.py	92.85% <0.00%> (ø)
filer/models/thumbnailoptionmodels.py	88.88% <0.00%> (ø)
filer/fields/multistorage_file.py	88.34% <33.33%> (ø)
filer/templatetags/filer_admin_tags.py	90.32% <33.33%> (ø)
filer/admin/clipboardadmin.py	94.11% <50.00%> (ø)
... and 12 more

... and 1 file with indirect coverage changes