Update uefi-services requirement from 0.12 to 0.21 in /bootloader

Updates the requirements on [uefi-services](https://github.com/rust-osdev/uefi-rs) to permit the latest version.
- [Changelog](https://github.com/rust-osdev/uefi-rs/blob/main/CHANGELOG.md)
- [Commits](rust-osdev/uefi-rs@uefi-services-v0.12.1...uefi-services-v0.21.0)

---
updated-dependencies:
- dependency-name: uefi-services
  dependency-type: direct:production
...

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

bootloader/Cargo.toml

@@ -19,4 +19,4 @@ elfloader = "0.14"
 log = { version = "0.4", default-features = false }
 arrayvec = { version = "0.7.0", default-features = false }
 uefi = { version = "0.15", features = ["exts"] }
-uefi-services = "0.12"
+uefi-services = "0.21"