`TestPulumixIsUpToDate` verifies the code in pulumix is up-to-date, but
the generated code includes a copyright range including the current
year, which now fails because of 2024. This commit pins the copyright
date in the template to 2023.
**Each commit is separately reviewable.**
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Adds the pulumix subpackage implementing strongly-typed
inputs, outputs, and combinators.
internal/cmd/gen-pux-applyn generates all four variants of `Apply` for
up to 8 parameters.
See individual commits for more details on each change.
Resolves#13586