Use 'cli' updater instead of python-apt

[jcapona]

Status	Ticket/Issue
Ready/Hold	Ticket

Main changes

Remove usage of python-apt in favor of running apt commands in a shell and parsing output.

• Update backend to make the 'legacy' updater backend the default.
• Remove custom mocks used for testing in the backend.
• Remove option to select updater backend in the frontend.
• Fix state of updater page on success to display the correct state.

Screenshots (feature, test output, profiling, dev tools etc)

[insert screenshots here]

Other notes (e.g. implementation quirks, edge cases, questions / issues)

Manual testing tips

Tag anyone who definitely needs to review or help

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.16352% with 22 lines in your changes missing coverage. Please review.

Project coverage is 76.86%. Comparing base (9214fd4) to head (c342863).

Files	Patch %	Lines
pt_os_web_portal/os_updater/backend.py	88.97%	14 Missing ⚠️
frontend/src/msw/handlers.ts	0.00%	4 Missing ⚠️
pt_os_web_portal/os_updater/updater.py	83.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
+ Coverage   74.53%   76.86%   +2.33%     
==========================================
  Files         186      183       -3     
  Lines        4182     3942     -240     
  Branches      420      428       +8     
==========================================
- Hits         3117     3030      -87     
+ Misses       1064      911     -153     
  Partials        1        1              

Flag	Coverage Δ
js-tests	85.32% <50.00%> (-0.37%)	⬇️
python-tests	71.19% <88.07%> (+3.52%)	⬆️

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