closes https://linear.app/tryghost/issue/PLG-15
- removed `internalLinking` GA labs flag
- renamed search providers to `flex` and `basic`
- keeps old search provider around as it can handle non-English languages unlike the faster flex provider
- updated `search` service to switch from `flex` to `basic` when the site's locale is not english
- bumped Koenig packages to switch from a feature flag for toggling internal linking features to the presence of the `searchLinks` function in card config
- updated tests to correctly switch between flex and basic providers in respective suites
