Fix CI tests for MSRV

Author: Dirbaio

embedded-hal-nb/Cargo.toml

@@ -15,6 +15,6 @@ repository = "https://github.com/rust-embedded/embedded-hal"
 embedded-hal = { version = "=1.0.0-alpha.9", path = "../embedded-hal" }
 nb = "1"
 
-[dev-dependencies.stm32f1]
-version = "0.15"
-features = ["stm32f103", "rt"]
+[dev-dependencies]
+cortex-m-rt = "=0.7.1"  # 0.7.2 bumped its MSRV higher than embedded-hal's
+stm32f1 = { version = "0.15", features = ["stm32f103", "rt"] }