docs: outdated macOS tools imply tier 2

[scpeters]

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Right after a brew doctor complaint that A newer Command Line Tools release is available. I noticed This is a Tier 2 configuration with a link to https://docs.brew.sh/Support-Tiers#tier-2. When I didn't see outdated macOS tools in the list of Tier 2 configurations, I started to wonder if there was another reason for the Tier 2 message, but then I found the code that generated that message and confirmed that is the reason. Also, I saw that Xcode Command Line Tools installed and up to date is listed as a requirement for Tier 1 in these docs, so I think this is correct.

[MikeMcQuaid]

Thanks!