MINIFICPP-2420 Fix GCC-13 warnings

[fgerlits]

Fix maybe-uninitialized warnings in JoltUtils.cpp and Expression.cpp. In the latter, these were caused by an uninitialized variable in the date library (inlined here), which is fixed in the latest, unreleased, version.

Also:

• add missing Py_INCREF(Py_None) calls in PythonScriptEngine.cpp, which caused a none_dealloc exception at exit;
• use Ubuntu 24.04 CI runner instead of 20.04 (24.04 uses gcc 13).

https://issues.apache.org/jira/browse/MINIFICPP-2420

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