Set clang-format to use C++20

It's currently failing on the spaceship operator, introduced in C++20

Change-Id: Ic389688fc7f1544df34538d14e7acb666a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/21500
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

.clang-format

diff --git a/.clang-format b/.clang-format
index a8b5db8..e1aed24 100644
--- a/.clang-format
+++ b/.clang-format
@@ -1,2 +1,2 @@
 BasedOnStyle: Chromium
-Standard: c++17
+Standard: c++20