Update stm32-metapac requirement from 14.0.0 to 15.0.0 (#884)

Updates the requirements on [stm32-metapac](https://github.com/embassy-rs/stm32-data) to permit the latest version.
- [Commits](https://github.com/embassy-rs/stm32-data/commits)

---
updated-dependencies:
- dependency-name: stm32-metapac
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

rtic-monotonics/Cargo.toml

@@ -51,15 +51,15 @@ nrf5340-net-pac = { version = "0.12.2", optional = true }
 nrf9160-pac = { version = "0.12.2", optional = true }
 
 # STM32
-stm32-metapac = { version = "14.0.0", optional = true }
+stm32-metapac = { version = "15.0.0", optional = true }
 
 # i.MX RT
 imxrt-ral = { version = "0.5.3", optional = true }
 
 [build-dependencies]
 proc-macro2 = { version = "1.0.36", optional = true }
 quote = { version = "1.0.15", optional = true }
-stm32-metapac = { version = "14.0.0", default-features = false, features = [
+stm32-metapac = { version = "15.0.0", default-features = false, features = [
     "metadata",
 ], optional = true }