Bug#1124051: webkit2gtk: Please upgrade build-dep to llvm/clang 21

[Jeremy Bícha]

Control: unblock 1124069 by -1

> We would like to remove llvm-toolchain-19 from the archive.
> Please update to 21.

webkit2gtk in Unstable & Experimental uses gcc instead of clang for
armhf now. It looks like the maintainer intentionally kept this bug
open but this bug is no longer a blocker for removing
llvm-toolchain-19.

Thank you,
Jeremy Bícha