Move rust-toolchain.toml to root dir #3171

Job	Run time
Integration tests (ubuntu-20.04, 1.21, .CLI.)	15m 52s
Unit tests (ubuntu-20.04, 1.21)	5m 53s
Build (ubuntu-latest, x86_64-unknown-linux-gnu, amd64)	2m 19s
Integration tests (ubuntu-20.04, 1.21, ^Test(([^C])\|(C[^L])\|(CL[^I])).*$)	13m 50s
Unit tests (ubuntu-22.04, 1.21)	6m 5s
Build (ubuntu-latest, aarch64-unknown-linux-gnu, arm64)	2m 56s
Integration tests (ubuntu-22.04, 1.21, .CLI.)	15m 36s
Build (macos-latest, x86_64-apple-darwin, amd64)	3m 22s
Integration tests (ubuntu-22.04, 1.21, ^Test(([^C])\|(C[^L])\|(CL[^I])).*$)	13m 16s
Build (macos-latest, aarch64-apple-darwin, arm64)	6m 28s
Build (windows-latest, x86_64-pc-windows-gnu, amd64)	7m 28s
	1h 33m 5s

Provide feedback