coot: use "pkgconf" instead of pkg-config

[YoshitakaMo]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source --verbose --keep-tmp ./Formula/<FORMULA>.rb, where FORMULA is the name of the formula you're submitting?
• Does your formula have no offenses with brew style /path/to/formula.rb?
• Does your formula pass brew audit --formula brewsci/bio/<FORMULA> --online --git --skip-style?
• Does your formula pass brew linkage --cached --test --strict brewsci/bio/<FORMULA> after manual installation?

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The dependency of "pkg-config" is now deplicated. Use "pkgconf" instead.