Start to remove a bunch of complicated 0.63 modal.Cls code

[freider]

Background:

• For v0.62 lookup compatibility, we used to create _Function objects for every method.
• When we deprecated v0.62, we still had this code in place even though the class-bound method was no longer backed by any "real" backend function. In the client it mostly served as an information carrier to let us hydrate the (client-only) instance bound _Function.

This PR removes the need for this complicated information carrier, reducing the risk of us accidentally calling the class-bound method.

This will significantly help with further simplification of the class code, and once 0.66 is deprecated we should be able to remove further complicated code as the method metadata will then always be served directly by the class service function rather than the Cls.

TODO:

• Test that this works with lookups of pre-0.67 classes
• Run integration tests

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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

[freider]

_is_web_endpoint is a method, so this was a weird assignment...

[erikbern]

Nice!!

[devennavani]

lgtm!