docs: use GFM footnote syntax for the macOS signing note

[claude]

Requested by Samuel Attard, David Sanders · Slack thread

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Converts the remaining faux footnote in the README (the macOS / Mac App Store signing note) to GFM footnote syntax. Before: a manual <sup>*</sup> marker with an italicized note line below the platform list. After: a [^macos] GFM footnote, matching the [^ia32] style introduced in #1935. Wording is unchanged. This is the follow-up @malept requested in his #1935 review.

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