Change phrasing for "default property value" diagnostic

[DanielRosenwasser]

In #43005, we added a diagnostic:

"Missing '=' before default property value.

But these aren't default property values - they're initializers. I guess they're "defaults" in the absence of any other values, but that's not really the right term.

I think it should be

"Expected '=' for property initializer.

or

"Missing '=' for property initializer.

[DanielRosenwasser]

(@JoshuaKGoldberg in case you're interested)

[roottool]

@DanielRosenwasser I guess Expected '=' for property initializer. is the best. Because I think unified words is better than non-unified words. The some error messages uses Unexpected or expected in #43005.

[JoshuaKGoldberg]

Looks like @a-tarasyuk has got it already 😄 , but thanks - makes sense!