clang-format: Enable --Wno-error=unknown for compat with older versions

This prevents errors when encountering options which have been defined in newer
versions of clang-format, and are invalid in the YAML for the old version.

Bump minimum supported clang-format version to 12 (where `--Wno-error=unknown`
was added).

Use clang-format 12 on CI (13 is not available yet on the Ubuntu 20.04 images).

misc/scripts/clang_format.sh

@@ -23,7 +23,7 @@ while IFS= read -rd '' f; do
     for extension in ${CLANG_FORMAT_FILE_EXTS[@]}; do
         if [[ "$f" == *"$extension" ]]; then
             # Run clang-format.
-            clang-format -i "$f"
+            clang-format --Wno-error=unknown -i "$f"
             # Fix copyright headers, but not all files get them.
             if [[ "$f" == *"inc" ]]; then
                 continue 2