BREAKING CHANGE: For consistency, `ZSH_THEME_RANDOM_QUIET` now needs to
be `true` if you want to silence the "Random theme loaded" message in the
`random` theme.

The wiki specified that 1 or true was valid, while the code just
checked whether it was set to any value. Being more strict makes
sure that we're consistent with the rest of the Settings.

Fixes #10534