Support Python 3.13, deprecate Python 3.8

[jvstme]

Checklists

Running dstack under 3.9-3.13

• Forbid installing the dstack and gpuhunt packages under 3.8
• (optional) Upgrade the codebase to use 3.9 features
• In the codebase, address removals and deprecations in 3.13, if any
• Run tests under 3.9-3.13 in GitHub Actions

Running dstack configurations under 3.9-3.13

• Build dstackai/base:py3.13-* docker images
• Pre-pull dstackai/base:py3.13-* in dstack packer templates, do not pre-pull dstackai/base:py3.8-*
• Allow specifying python: 3.13 in run configurations, deprecate specifying python: 3.8

After a few dstack releases

• Stop building dstackai/base:py3.8-* docker images
• Forbid specifying python: 3.8 in run configurations

[jvstme]

Running dstack under 3.13 seems to be blocked by MagicStack/asyncpg#1181. Other steps in this issue shouldn't be blocked

[un-def]

asyncpg v0.30.0 with Python 3.13 support has just been released.

[jvstme]

Running dstack under 3.13 is also blocked by:

• Cannot define models with Annotated under Python 3.13 zmievsa/pydantic-duality#15
• Cannot import client libraries under Python 3.13 googleapis/google-cloud-python#13187

[zmievsa]

Let me take care of the first issue.