chore: update gemini styleguide to c++17 (googleapis#16082)

Author: scotthart

.gemini/styleguide.md

@@ -3,7 +3,7 @@ https://google.github.io/styleguide/cppguide.html.
 
 This repository differs from the Google C++ style guide in the following ways:
 
-- supports C++14 as its minimum C++ standard version.
+- supports C++17 as its minimum C++ standard version.
 - lists local includes first, then system includes.
 - uses std::mutex instead of absl::mutex.
 - uses google::cloud::future and google::cloud::promise instead of std::future