format-code: Update default clang-format to v11

v6 is a bit out of date and not what the infrastructure uses, so move
with the times.

Change-Id: If0692bbe6809e1dcbd951b6cfa737341cf4246f0
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/format-code.sh b/format-code.sh
index b3ddcf6..0f67180 100755
--- a/format-code.sh
+++ b/format-code.sh
@@ -8,7 +8,7 @@
 
 # Use the provided clang-format, only define a version
 # if we don't have one provided already
-export CLANG_FORMAT="${CLANG_FORMAT:-clang-format-6.0}"
+export CLANG_FORMAT="${CLANG_FORMAT:-clang-format-11}"
 
 # phosphor-mboxd is a fork of mboxbridge, the reference mbox daemon
 # implementation. mboxbridge is C written with the style of the Linux kernel.