feat!: remove deprecated dev.startUrl

[chenjiahan]

Summary

Remove the deprecated dev.startUrl config, use server.open instead.

Reference

#2508

Changes:

• Rename dev.startUrl to server.open.
• Rename dev.beforeStartUrl to server.open.before.

Motivation:

• Align with Rspack dev server's open config.
• Allow to use in the preview server.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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