feat: update openapi-generator version to 7.2.0 for elixir

OpenAPI Generator version 7+ picks up https://github.com/OpenAPITools/openapi-generator/pull/16061 which changes the underlying decoding library which is better suited to Phoenix projects and is actively maintained.

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 approval (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 the necessary documentation within the code base (if appropriate).