fix: remove DeprecationWarning json_encoder

[utnapischtim]

• The flask.json.JSONEncoder is set deprecated with Flask v2.2 and will be removed with Flask v2.3.

• Flask.json.JSONEncoder was inherited from json.JSONEncoder. This is the reason to use that class as the new parent. Flask.json.JSONEncoder didn't added something to the constructor, therfore json.JSONEncoder is a save parent.

• json.JSONEncoder is not enough. Flask.json.JSONEncoder handled some special types with a specialized _default function. This _default method is used also in this commit to ensure the same behaviour as before.

[kpsherva]

@ppanero having in mind your latest changes for the serializers, could you check this PR ?

[fenekku]

As coordinator + person with merge ability:

• this is marked with high priority
• has been checked by 2 people
• is understood change
• has no reason to interfere with other work
• is easy to revert

ergo, I will merge this in. Thanks @utnapischtim !