This has become much easier now. Search for cppstandard in your vs code extension settings and choose the version of C++ you want the extension to use from the drop down.

In order to make sure your debugger is using the same version, make sure you have something like this for your tasks.json, where the important lines are the --std and the line after that defines the version.

{
  "tasks": [
    {
      "type": "cppbuild",
      "label": "C/C++: g++ build active file",
      "command": "/usr/bin/g++",
      "args": [
        "-std=c++17",
        "-I",
        "${fileDirname}",
        "-g",
        "${fileDirname}/*.cpp",
        "-o",
        "${workspaceFolder}/out/${fileBasenameNoExtension}.o"
      ],
      "options": {
        "cwd": "${workspaceFolder}"
      },
      "problemMatcher": ["$gcc"],
      "group": {
        "kind": "build",
        "isDefault": true
      }
    }
  ],
  "version": "2.0.0"
}

Note that if you're copying the above tasks.json directly, you'll need to have a folder named out in your workspace root.