JANITORIAL: Use C++03 standard in clang-format rules

This mostly addresses the problem of ending double templates with `> >`
instead of `>>`, a feature only enabled in newer C++ standards.

.clang-format

@@ -19,4 +19,5 @@
     SpacesInAngles: false,
     SpacesInParentheses: false,
     SpacesInSquareBrackets: false,
+    Standard: c++03,
 }