bump-formula-pr: fix type of detected version

[EricFromCanada]

Relates to recent type signature additions, e.g. in #16886. Fixes #16917.

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[EricFromCanada]

Error is unrelated:

 Error: GitHub::search_issues queries GitHub issues with the passed parameters

Failure/Error: expect(results.first["title"]).to eq("Shall we run `brew update` automatically?")

  expected: "Shall we run `brew update` automatically?"
       got: "Shall we move more things to taps?"

[apainintheneck]

That error got fixed on the master branch so this should be good to go after a rebase.

[MikeMcQuaid]

Thanks @EricFromCanada!