More explicit support tiers

[MikeMcQuaid]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

We currently have two tiers of support:

• "supported"
• "unsupported"

In reality, we have a bunch more nuance:

• Supported platforms e.g. latest macOS version on Apple Silicon
• Soon-to-be-supported platforms e.g. ARM64 Linux, macOS prereleases
• One-day-unsupported platforms e.g. one-too-old macOS versions, likely Apple x86_64 one day
• Somewhat unsupported platforms e.g. non-default prefixes, old macOS versions

What is the motivation for the feature?

Make it more obvious to users what the support state is for a given platform by defining specific support tiers which are both documented and communicated in different ways:

• Tier 1 e.g. macOS Sequoia (default prefix), Linux x86_64 (default prefix) with newish system libc: full CI coverage, full support, no warnings
• Tier 2 e.g. any non-default prefix), ARM64 Linux (today), new macOS prereleases, Linux with older glibc: partial/no CI coverage, partial support, brew doctor warnings, no other runtime warnings
• Tier 3 e.g. macOS 12, ARM64 (6 months ago): no support, no CI coverage, runtime warnings at brew install time, brew doctor warnings
• Unsupported e.g. macOS 10.6, i686, ppc64: things are actively expected to be broken, we don't allow any runs, Homebrew needs patched or e.g. Tigerbrew/MacPorts should be used instead

How will the feature be relevant to at least 90% of Homebrew users?

• Clearer indication about what is supported now and what will be in future
• Ability to warn users before something changes support tier

What alternatives to the feature have been considered?

• The status quo of "supported" and "unsupported"
• More aggressively dropping support or refusing to run on lower tier/unsupported platforms

CC @Bo98 as he had this initial idea