fix(*): pin rust to 1.71.0 until compile time regression fixed

Rust 1.72.0 introduced compile time regressions for async heavy code:

rust-lang/rust#115283

Reverting to 1.71.0 gets us back to reasonable compile times. We should
stay there until a fix is released.

rust-toolchain

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "stable"
+channel = "1.71.0"
 profile = "default"