use non-deprecated way of changing JSON encoder

[briantist]

There were warnings about using app.json_encoder in Flask (it will be removed in 2.3).

This PR updates the way of doing that to the new supported way.

[codecov]

Codecov Report

Base: 52.83% // Head: 52.94% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (f636f26) compared to base (52306a4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   52.83%   52.94%   +0.10%     
==========================================
  Files          22       22              
  Lines         899      901       +2     
==========================================
+ Hits          475      477       +2     
  Misses        424      424              

Impacted Files	Coverage Δ
galactory/__init__.py	44.59% <100.00%> (+1.53%)	⬆️

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