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.

Some of the Python templates had references to {{#disallowAdditionalPropertiesIfNotPresent}}. It turns out that this check behaves differently if users leave disallowAdditionalPropertiesIfNotPresent unspecified than it does if users explicitly configure disallowAdditionalPropertiesIfNotPresent: true. That means that swapping the default would be a breaking change for Python generators. To work around that, I've introduced a new legacyDisallowAdditionalPropertiesDefaultBehavior flag that is used by the impacted Python generators. Users who were using the impacted generators and were not explicitly configuring disallowAdditionalPropertiesIfNotPresent can continue to generate equivalent code by setting these options:

disallowAdditionalPropertiesIfNotPresent: true
legacyDisallowAdditionalPropertiesDefaultBehavior: true

Closes #21169
Closes #21121, closes #21446 (by making expected behavior documented in those issues the default rather than requiring explicit config)
Closes #16575 (duplicate)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.