fix: Remove critical-section-single-core feature for Hubris compatibility

- Remove critical-section-single-core feature from cortex-m dependency
- Prevents conflicts with Hubris's own critical section implementation
- Allows DDK to be used as a library within Hubris tasks
- Follows standard Hubris ecosystem patterns
- Maintains cortex-m 0.7.5 for low-level ARM Cortex-M functionality

Author: rusty1968

Cargo.lock

@@ -24,7 +24,7 @@ fugit = "0.3.7"
 proposed-traits = { git = "https://github.com/rusty1968/proposed_traits.git", package = "proposed-traits", rev = "43fe54addf323dc17915f1fc5f991f9d94eb161a" }
 hex-literal = "0.4"
 
-cortex-m = { version = "0.7.5", features = ["critical-section-single-core"] }
+cortex-m = { version = "0.7.5" }
 cortex-m-rt = { version = "0.6.5", features = ["device"] }
 panic-halt = "1.0.0"