Fix caching offers with GPU requirements (#2210)

Use `Requirements.json` instead of
`Requirements.dict`+`json.dumps`, which fails on
enums.

Also remove `sort_keys=True`, which seems to be
redundant.

> field order is preserved by .dict() and .json()
> etc.
https://docs.pydantic.dev/1.10/usage/models/#field-ordering

src/dstack/_internal/core/backends/base/compute.py

@@ -1,4 +1,3 @@
-import json
 import os
 import re
 import threading
@@ -188,7 +187,7 @@ def _get_offers_cached_key(self, requirements: Optional[Requirements] = None) ->
         # Requirements is not hashable, so we use a hack to get arguments hash
         if requirements is None:
             return hash(None)
-        return hash(json.dumps(requirements.dict(), sort_keys=True))
+        return hash(requirements.json())
 
     @cachedmethod(
         cache=lambda self: self._offers_cache,