[test][prefix] Update CXX algorithm minimal standard to C++14

It has been the case for a while for beat/beat.backend.cxx>
4 jobs for 99_ensure_output_error_is_reported in 25 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #202849
docker-build
build_linux_36

00:14:31

64.0%
passed #202850
docker-build
build_linux_37

00:14:29

64.0%
passed #202847
macosx
build_macosx_36

00:25:04

64.0%
passed #202848
macosx
build_macosx_37

00:24:47

64.0%