Fix migrations using deprecated migration plugin

I was working on #2890690: MigrateLookup plugin has inconsistent return values. and discovered a usage of the deprecated plugin 'migration'. So looked for more:

$ grep -rw "plugin: migration" core/modules/*/migrations
core/modules/content_translation/migrations/d6_taxonomy_term_localized_translation.yml:    plugin: migration
core/modules/content_translation/migrations/d6_taxonomy_term_translation.yml:    plugin: migration
core/modules/content_translation/migrations/d6_taxonomy_term_translation.yml:      plugin: migration

They look familiar. And yes I worked on those issues and didn't update the migration, it must have been copy pasted a long time ago.
#2784371: Migrate D6 i18n taxonomy term language (but not yet translations)
#2886609: Migrate translations for D6 i18n taxonomy 'localized' terms

Attached is an untested fix