Default to OAS-compliant handling of additionalProperties

[ctreatma]

This changes the default value for disallowAdditionalPropertiesIfNotPresent from true to false, which means that the generator will now, by default, produce code that complies OpenAPI standards with respect to the handling of additionalProperties.

Note that a number of example configs did not have to be updated as a result of this change, which indicates that the disallowAdditionalPropertiesIfNotPresent option had no effect on the generators used in those examples.

Closes #21169 Closes #16575 (duplicate)

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[ctreatma]

@wing328 When you have a moment, please take a look. To provide a more complete fallback for this change, I added a new config flag for Python generators to maintain pre-existing behavior around the disallowAdditionalPropertiesIfNotPresent option. Let me know if you'd like to see this handled differently.