Update rtt-target requirement in /rtic_v1/stm32l4_heartbeat

Updates the requirements on [rtt-target](https://github.com/probe-rs/rtt-target) to permit the latest version.
- [Release notes](https://github.com/probe-rs/rtt-target/releases)
- [Commits](probe-rs/rtt-target@v0.3.1...v0.3.1)

---
updated-dependencies:
- dependency-name: rtt-target
  dependency-type: direct:production
...

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

Author: dependabot[bot]

rtic_v1/stm32l4_heartbeat/Cargo.toml

@@ -19,7 +19,7 @@ stm32l4xx-hal = { version = "0.7.1", features = ["rt", "stm32l412"] }
 heapless = "0.7.10"
 systick-monotonic = "1.0.0"
 panic-rtt-target = { version = "0.1.2", features = ["cortex-m"] }
-rtt-target = { version = "0.3.1", features = ["cortex-m"] }
+rtt-target = { version = "0.4.0", features = ["cortex-m"] }
 
 # this lets you use `cargo fix`!
 [[bin]]