chore(deps): update heapless requirement from 0.8 to 0.9

Updates the requirements on [heapless](https://github.com/rust-embedded/heapless) to permit the latest version.
- [Release notes](https://github.com/rust-embedded/heapless/releases)
- [Changelog](https://github.com/rust-embedded/heapless/blob/main/CHANGELOG.md)
- [Commits](rust-embedded/heapless@v0.8.0...v0.9.1)

---
updated-dependencies:
- dependency-name: heapless
  dependency-version: 0.9.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

rtic-sync/Cargo.toml

@@ -18,7 +18,7 @@ repository = "https://github.com/rtic-rs/rtic"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-heapless = "0.8"
+heapless = "0.9"
 critical-section = "1"
 rtic-common = { version = "1.0.0", path = "../rtic-common" }
 portable-atomic = { version = "1", default-features = false }