Updated clarification on Supported Platforms

[sethladd]

What information needs to be added?

This is a tracking issue to take a look at our Supported Platforms page https://docs.flutter.dev/reference/supported-platforms and see if there are opportunities to communicate levels (tiers?) of support in addition to how much we're automatically testing.

Some prior art is here for study: https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html (I'm sure there are plenty more prior arts out there)

Where should this new content appear?

No response

I would like to fix this problem.

• [ ] I will try and fix this problem on docs.flutter.dev.