build(deps): update addr2line requirement from 0.16 to 0.21 in /kernel

Updates the requirements on [addr2line](https://github.com/gimli-rs/addr2line) to permit the latest version.
- [Changelog](https://github.com/gimli-rs/addr2line/blob/master/CHANGELOG.md)
- [Commits](gimli-rs/addr2line@0.16.0...0.21.0)

---
updated-dependencies:
- dependency-name: addr2line
  dependency-type: direct:production
...

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

kernel/Cargo.toml

@@ -52,7 +52,7 @@ gimli = { version = "0.26", default-features = false, features = ["read", "endia
 gdbstub = { git = "https://github.com/daniel5151/gdbstub.git", branch = "dev/0.6", default-features = false, features = ["alloc"] }
 gdbstub_arch = { git = "https://github.com/daniel5151/gdbstub.git", branch = "dev/0.6" }
 # Optional external libraries:
-addr2line = { version = "0.16", default-features = false, features = ["rustc-demangle"], optional = true }
+addr2line = { version = "0.21", default-features = false, features = ["rustc-demangle"], optional = true }
 smoltcp = { version = "0.8.0", default-features = false, features = [ "alloc", "log", "proto-ipv4", "proto-igmp", "proto-dhcpv4", "socket-raw", "socket-icmp", "socket-udp", "socket-tcp" ], optional = true }
 
 [[bin]]