Remove support for when: section in typed processes

[bentsherman]

This PR removes support for when: in typed processes.

While it was preserved during the preview period to ease migration, the stable version of typed processes should not support it

Aside from being an anti-pattern, when: can actually break semantic guarantees of dataflow logic. If a process is called with a dataflow value and returns a dataflow value, and I use when: to disable that process via config, it will return an "empty" dataflow value which is invalid

Rather than deprecating it across the board, it will remain allowed in legacy processes and removed in typed processes. Migrating to static typing is the best point at which to also migrate away from when: because records and type checking makes it easier to do

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[pditommaso]

I'm not totally convinced about removing this. I was even considering that this could implement a similar pattern as until/recursion

[bentsherman]

when: is not compatible with static typing. I don't see how recursion is relevant here