How to create my own custom simple action in D8?
Action plugin yaml definitions need to be prefixed with the name system.action.action_name.yml, and not mymodule.action_name.yml.

For reference, check the provided action plugins for node and user module in core in their config/install and config/schema folders.

Uninstall the module, rename the files, and install the module again so the changes are picked up. You should then see your custom action.

So:

config/install/system.action.resolve_news_migration_flag.yml