fix: generate with dart instead of dart-dio [dart]

[AllenKaplan]

Description

Current ory sdk for dart generation uses dart-dio which does not support null safety intrduced in Dart 2.12. As Flutter 3 requires Dart's null safety, it is a blocking issue for the ory sdk to not support Flutter 3. The change uses the "default" Dart generator dart instead of dart-dio

Related Issue or Design Document

Ory & Dart's null safety is discussed here https://github.com/ory/cloud/issues/13

Checklist

• [X] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
• [X] I have read the security policy.
• [X] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got green light (please contact security@ory.sh) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added necessary documentation within the code base (if appropriate).

Further comments

Full disclosure I have not tested generating Ory SDK with dart over dart-dio 🙊

[AllenKaplan]

Awesome, thank you for your contribution! I've got one question though :)

@aeneasr responded :)