Both
https://docs.magnolia-cms.com/content-translation-support-ext/index.html
and
https://docs.magnolia-cms.com/content-translation-support-ext/legacy.html
refer to:
info.magnolia.translation.ext.core.translation.finder.AdaptivePropertiesToTranslateFinder

The correct class name is actually:
info.magnolia.translation.ext.core.finder.AdaptivePropertiesToTranslateFinderExt
See: https://git.magnolia-cms.com/projects/SERVICES/repos/content-translation-support-ext/browse/magnolia-content-translation-support-ext-core/src/main/java/info/magnolia/translation/ext/core/finder/AdaptivePropertiesToTranslateFinderExt.java