Rollup merge of #144906 - Kobzol:infra-team-tier-bump, r=davidtwco

Require approval from t-infra instead of t-release on tier bumps

Discussed at https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Tier.201.20target.20promotion.20RFC.20FCP.20sign-offs/with/532735844.

I also changed "viability and value" to just "viability". I think that t-infra should decide whether it's viable to support a given target on our CI. The value should be determined by t-compiler.

r? ``@jieyouxu``

Author: jieyouxu