ENH -- Swap `"AZURE_OPENAI_KEY"` for `"AZURE_OPENAI_API_KEY"`

[it176131]

In the Microsoft docs they mention setting the environment variable to "AZURE_OPENAI_API_KEY" rather than "AZURE_OPENAI_KEY".

This is also reflected in langchain's implementation.

Users could have the same key with two different variable names, but I think it'd be easier if we switched to the recommended name in the MS docs.

# from https://github.com/explosion/spacy-llm/blob/117f68963870fd2a4af4c706c40cf223c6ae6fde/spacy_llm/models/rest/azure/model.py#L62
    @property
    def credentials(self) -> Dict[str, str]:
        # Fetch and check the key
-       api_key = os.getenv("AZURE_OPENAI_KEY")
+       api_key = os.getenv("AZURE_OPENAI_API_KEY")
        if api_key is None:
            warnings.warn(
                "Could not find the API key to access the Azure OpenAI API. Ensure you have an API key "
                "set up (see "
                "https://learn.microsoft.com/en-us/azure/ai-services/openai/quickstart?pivots=rest-api&tabs=bash#set-up"
                ", then make it available as an environment variable 'AZURE_OPENAI_KEY'."
            )