Rollup merge of #121994 - wesleywiser:update_musl_version_docs, r=ehuss

Update platform-support.md with supported musl version

This just reflects the current status quo, there is no actual change here since the update to musl 1.2.3 occurred in #107129 and was approved in rust-lang/compiler-team#572.

I also normalized all mentions of musl libc to "musl" (non-capitalized per the project's site and Wikipedia page).

r? ``@ehuss``

Author: matthiaskrgr