tests: bump the C++ std for protobuf tests

Based on https://opensource.google/documentation/policies/cplusplus-support
Google no longer supports C++11, and protobuf spawns an `#error` if you
don't have at least 14.

So, perform our currently scheduled automatic bump.

(cherry picked from commit a8b9bd2b74)

test cases/frameworks/5 protocol buffers/meson.build

@@ -1,4 +1,4 @@
-project('protocol buffer test', 'cpp', default_options: ['cpp_std=c++11'])
+project('protocol buffer test', 'cpp', default_options: ['cpp_std=c++14'])
 
 protoc = find_program('protoc', required : false)
 dep = dependency('protobuf', required : false)