update Standard in .clang-format

GN currently uses C++17.

ref: https://clang.llvm.org/docs/ClangFormatStyleOptions.html#:~:text=Won%27t%20be%20affected-,Standard,-(LanguageStandard)

Bug: chromium:1309032
Change-Id: I49db5c2681e4dc2e69c82cb80c567ec67152dd7c
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/13322
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
diff --git a/.clang-format b/.clang-format
index b17a52a..a8b5db8 100644
--- a/.clang-format
+++ b/.clang-format
@@ -1,2 +1,2 @@
 BasedOnStyle: Chromium
-Standard: Cpp11
+Standard: c++17