Turn stable-features error into a warning

This CL turns the stable-features lint into a warning.  This is
necessary to complete toolchain updates until we finish enforcing the
"no unstable features policy".

Test: ./test_compiler --prebuilt-path ../../dist/rust-1.66.1.tar.gz
Bug: 267698452
Bug: 263153841
Change-Id: Ib095dea045111ad3d89327a694aa5fa5b7f3128c