feat: Replace PREPARING status with CREATING for compute session and kernel

[fregataa]

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[fregataa]

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• feat: Replace PREPARING status with CREATING for compute session and kernel #2883 👈
• feat: Add new sessions & kernels status #2647 : 1 other dependent PR (#2692 )
• refactor: image configuration when create kernels #2646
• feat: Add session status checker API #2312 : 1 other dependent PR (#2836 )
• refactor: revamp update mechanism of session & kernel status #2311
• feat: Add interface that update session and kernel status #2581
• main

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