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/hooks/pre-commit-clang-format

@@ -76,7 +76,7 @@ fi
 
 # To get consistent formatting, we recommend contributors to use the same
 # clang-format version as CI.
-RECOMMENDED_CLANG_FORMAT_MAJOR_MIN="11"
+RECOMMENDED_CLANG_FORMAT_MAJOR_MIN="12"
 RECOMMENDED_CLANG_FORMAT_MAJOR_MAX="13"
 
 if [ ! -x "$CLANG_FORMAT" ] ; then
@@ -146,7 +146,7 @@ do
     #    +++ - timestamp
     # to both lines working on the same file and having a/ and b/ prefix.
     # Else it can not be applied with 'git apply'.
-    "$CLANG_FORMAT" -style=file "$file" | \
+    "$CLANG_FORMAT" -style=file "$file" --Wno-error=unknown | \
         diff -u "$file" - | \
         sed -e "1s|--- |--- a/|" -e "2s|+++ -|+++ b/$file|" >> "$patch"
 done

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