Devcontainer: Update to llvm 20

Author: https://github.com/ADKaster
Commit: 30d01f7920
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4736

.devcontainer/devcontainer.json

@@ -6,7 +6,7 @@
         "ghcr.io/devcontainers/features/github-cli:1": {},
         "ghcr.io/devcontainers-contrib/features/pre-commit:2": {},
         "./features/ladybird": {
-            "llvm_version": 19
+            "llvm_version": 20
         },
         "./features/vcpkg-cache": {
             "release_triplet": true,

.devcontainer/features/ladybird/devcontainer-feature.json

@@ -18,12 +18,12 @@
         "llvm_version": {
             "type": "string",
             "proposals": [
-                17,
                 18,
                 19,
+                20,
                 "trunk"
             ],
-            "default": 19,
+            "default": 20,
             "description": "Select LLVM compiler version to use"
         }
     }