Update .bazelrc to use C++ 17

This is in preparation for moving upb into the protobuf repo. Upb requires C++
17. Protobuf supports C++ 14, and our non-upb CI tests will keep using C++ 14
to ensure that it still works. Our Bazel CI runs use the bazelrc files in the
ci/ directory, so these runs should be unaffected by this change.

PiperOrigin-RevId: 555223181

.bazelrc

@@ -1,4 +1,4 @@
-build --cxxopt=-std=c++14 --host_cxxopt=-std=c++14
+build --cxxopt=-std=c++17 --host_cxxopt=-std=c++17
 
 build:dbg --compilation_mode=dbg