I have the problem mentioned in the title. You could say that this thread duplicates another one: How do I turn off error validation for annotations in IntelliJ IDEA?
Bu
A simpler solution - inline
Put the caret on the @Override
word and move the caret on the left side until the red bulb icon will appear. Then click on it.
Click on Set language level to 6 - Override in interfaces
The method above is an alternative to the following approach:
Go to File > Project Structure... or press Ctrl+Alt+Shift+S
Go to Project Settings > Modules > Sources > Language level and choose any level that is 6 or greater than 6.