Auto merge of #153684 - cuviper:min-llvm-21, r=nikic

Update the minimum external LLVM to 21

With this change, we'll have stable support for LLVM 21 and 22.
For reference, the previous increase to LLVM 20 was rust-lang/rust#145071.

cc @rust-lang/wg-llvm
r? nikic

Author: bors