plugdata@nightly: deprecate

[bevanjkay]

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After making any changes to a cask, existing or new, verify:

• The submission is for a stable version or documented exception.
• brew audit --cask --online plugdata@nightly is error-free.
• brew style --fix plugdata@nightly reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused (add your cask's name to the end of the search field).
• brew audit --cask --new plugdata@nightly worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask plugdata@nightly worked successfully.
• brew uninstall --cask plugdata@nightly worked successfully.

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes, including zap stanza paths.

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Deprecating because the cask fails the macOS gatekeeper check