Fix `allowed_search_types` in `VectorStoreRetriever` (#8064)

Unexpectedly changed at
6792a3557d

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I guess `allowed_search_types` is unexpectedly changed in
6792a3557d,
so that we cannot specify `similarity_score_threshold` here.

```python
class VectorStoreRetriever(BaseRetriever):
    ...
    allowed_search_types: ClassVar[Collection[str]] = (
        "similarity",
        "similarityatscore_threshold",
        "mmr",
    )

    @root_validator()
    def validate_search_type(cls, values: Dict) -> Dict:
        """Validate search type."""
        search_type = values["search_type"]
        if search_type not in cls.allowed_search_types:
            raise ValueError(...)
        if search_type == "similarity_score_threshold":
            ... # UNREACHABLE CODE
```

VectorStores Maintainers: @rlancemartin @eyurtsev

langchain/vectorstores/base.py

@@ -465,7 +465,7 @@ class VectorStoreRetriever(BaseRetriever):
     search_kwargs: dict = Field(default_factory=dict)
     allowed_search_types: ClassVar[Collection[str]] = (
         "similarity",
-        "similarityatscore_threshold",
+        "similarity_score_threshold",
         "mmr",
     )