Remove postgresql-specific default values in model definition

[whhe]

Summary

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Resolves #15411

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Checklist

Important

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• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

I think this will introduce breaking changes to our existing system, please provide a fallback plan as well.

[whhe]

I think this will introduce breaking changes to our existing system, please provide a fallback plan as well.

This change is not expected to introduce changes to the table structure. Which part do you think may break the existing design?

[crazywoola]

@laipz8200 Thoughts?

[laipz8200]

will check it later