set up the Minimum Supported Rust Version

[Geal]

trafficstars

Since the router is usable as a library, we should:

• find out which minimal Rust version we can build the router on
• which minimal Rust version we want the router to build on
• set up a CI pass to guarantee it
• add a badge in the README

[SimonSapin]

We already have rust-toolchain.toml that makes CI use a fixed toolchain version (1.61.0 today) rather than the Stable channel (though see https://github.com/apollographql/router/issues/1313). I agree it would be good to choose and document a policy for when we can change that version. As long as the router is in 0.x I think bumping whenever is fine. For 1.0+, we should what constraints our users might or might not have.

Finding what minimum version happens to compile today is mildly interesting but I feel that keeping it as low as possible should not be a goal for its own sake.