GIT_SILENT: use c++14 everywhere

2 jobs for master in 11 minutes and 35 seconds (queued for 162 minutes and 1 second)
Status Job ID Name Coverage
  Build
passed #52228
build_clazy_clang_tidy

00:06:40

passed #52227
linux kf5-qt5 qt5.12

00:04:55

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