Fix schema warnings and errors for Webhooks

[zhiltsov-max]

Motivation and context

• Removed extra update methods from schema
• Fixed Webhooks endpoints
• Fixed Webhooks enum names

How has this been tested?

Checklist

• [ ] I submit my changes into the develop branch
• [ ] I have added a description of my changes into CHANGELOG file
• [ ] I have updated the documentation accordingly
• [ ] I have added tests to cover my changes
• [ ] I have linked related issues (read github docs)
• [ ] I have increased versions of npm packages if it is necessary (cvat-canvas, cvat-core, cvat-data and cvat-ui)

License

• [ ] I submit my code changes under the same MIT License that covers the project. Feel free to contact the maintainers if that's a concern.

[zhiltsov-max]

/check

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