Update Flask JSON interfaces

[thomas-bc]

Originating Project/Creator
Affected Component
Affected Architectures(s)	Flask JSON
Related Issue(s)	nasa/fprime#2077
Has Unit Tests (y/n)
Builds Without Errors (y/n)
Unit Tests Pass (y/n)
Documentation Included (y/n)

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Change Description

Updates Flask JSON interfaces to the newer pattern.

Rationale

Flask deprecated and removed the older way of doing things. See nasa/fprime#2077

From Flask release notes:

Setting custom json_encoder and json_decoder classes on the app or a blueprint, and the corresponding json.JSONEncoder and JSONDecoder classes, are deprecated. JSON behavior can now be overridden using the app.json provider interface. pallets/flask#4692

[thomas-bc]

Would allow to resolve the Dependabot alert on a Flask vulnerability: https://github.com/nasa/fprime/security/dependabot/11