Enforce consistent string literal quoting style with single quotes

[robertcheramy]

We currently have a mixed string literal quoting style. The overwhelming majority of the code use single quotes, so I plan to enforce single quote with rubocop. This can inly be done when we have no open PRs because it could produce merge conflicts.

Rubystyle: https://rubystyle.guide/#consistent-string-literals-single-quote
Rubocop: https://docs.rubocop.org/rubocop/cops_style.html#stylestringliterals

[robertcheramy]

Moving to milestone 0.32 - this is a big change and I don't want to do it shortly before a release.