refactor(gem): favor built-in zsh completion for `gem` (#12576)

Completion for `gem` was included in zsh 5.5 and newer. This change
only uses the Oh My Zsh one if running an older version.

Reference: https://github.com/zsh-users/zsh/commit/9881778d48caf842f794d986bc9befebf0fb01c2