Add modal.FunctionCall.gather to replace modal.functions.gather

[mwaskom]

Describe your changes

Moving gather to be a staticmethod on modal.FunctionCall makes it more discoverable and clarifies its purpose. Because we now import modal.FunctionCall into the top-level namespace, we no longer need to recommend using names directly from modal.functions, which helps enforce a clearer separation between public / private API.

• Closes CLI-334

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Changelog

• We've moved the modal.functions.gather function to be a staticmethod on modal.FunctionCall.gather. The former spelling has been deprecated and will be removed in a future version.

[freider]

I like it!