Remove the old backup API

The old API is incapable of expressing good backup semantics. We'll throw it away and add a new one. This also comes with an API version bump reflecting the incompatibility of this change.

Created by: codecov[bot]

Codecov Report

Merging #294 (517a330e) into main (e81dbcf0) will decrease coverage by 0.32%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   91.76%   91.44%   -0.33%     
==========================================
  Files          51       51              
  Lines        4689     4569     -120     
  Branches      645      630      -15     
==========================================
- Hits         4303     4178     -125     
- Misses        338      342       +4     
- Partials       48       49       +1     

Impacted Files	Coverage Δ
src/_zkapauthorizer/_plugin.py	71.66% <100.00%> (ø)
src/_zkapauthorizer/api.py	100.00% <100.00%> (ø)
src/_zkapauthorizer/config.py	100.00% <100.00%> (ø)
src/_zkapauthorizer/model.py	94.68% <100.00%> (-0.18%)
src/_zkapauthorizer/resource.py	98.66% <100.00%> (-1.34%)
src/_zkapauthorizer/storage_common.py	94.02% <100.00%> (+0.09%)
src/_zkapauthorizer/tests/strategies.py	94.66% <100.00%> (+0.02%)
src/_zkapauthorizer/tests/test_client_resource.py	97.02% <100.00%> (-1.42%)
src/_zkapauthorizer/tests/test_model.py	93.13% <100.00%> (-0.18%)
src/_zkapauthorizer/tests/test_plugin.py	87.74% <100.00%> (+0.06%)
... and 7 more

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