[libc++][CI] Upgrade LLVM HEAD version in Docker image (#108774)

Before changing the compiler version in
#108761, we first of all need
to upgrade the HEAD version to `Clang-20` in the Docker files and push
new builder images to the CI.

Author: robincaloudis

libcxx/utils/ci/docker-compose.yml

@@ -2,7 +2,7 @@ version: '3.7'
 
 x-versions: &compiler_versions
   GCC_LATEST_VERSION: 14
-  LLVM_HEAD_VERSION: 19
+  LLVM_HEAD_VERSION: 20
 
 services:
   buildkite-builder: