feat!: remove deprecated backup window flag

[phm07]

This flag has been deprecated (and not working) since 2018: #147

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.14%. Comparing base (f2fb321) to head (2bddf46).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #710      +/-   ##
==========================================
- Coverage   60.14%   60.14%   -0.01%     
==========================================
  Files         191      191              
  Lines        6808     6802       -6     
==========================================
- Hits         4095     4091       -4     
+ Misses       2099     2098       -1     
+ Partials      614      613       -1     

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[apricote]

This is a potentially breaking change. Even though this flag shows a warning since 2018, we should try to avoid making breaking changes on purpose.

I have created a v2 milestone and added this PR to it, so we have a list of things to tackle if we decide to work on it.