Enable string-quotes as single
Sets string-quotes: "single" in stylelint (and transitions to set StringQuotes as single_quotes, which is the default of scss-lint).
We need to set StringQuotes of scss-lint as the default of @gitlab/stylelint-config (stylelint) is now string-quotes: "single".
Configuration of @gitlab/stylelint-config: https://gitlab.com/gitlab-org/frontend/gitlab-stylelint-config/-/blob/052570fdbf85814a6877e54e04a3367ab234cc38/index.js#L81
Upstream docs: https://stylelint.io/user-guide/rules/string-quotes
Edited by Takuya Noguchi