config: fix acceptance of storage_quota 0, fixes #8499

[ThomasWaldmann]

We don't want to accept very small quota limits < 10M, but 0 (quota disabled) is acceptable.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.60%. Comparing base (2230932) to head (e74c912).
Report is 12 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/archiver.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #8595      +/-   ##
=============================================
+ Coverage      82.24%   82.60%   +0.35%     
=============================================
  Files             38       38              
  Lines          11076    11083       +7     
  Branches        2130     1714     -416     
=============================================
+ Hits            9110     9155      +45     
+ Misses          1390     1372      -18     
+ Partials         576      556      -20     

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