BUG: DataFrame[Int64].mean().dtype is object, should be Float64
This breaks the last corner case I want to test for #33036.

```
arr = np.random.randn(4, 3).astype("int64")
df = pd.DataFrame(arr).astype("Int64")
df.iloc[:, 1] = pd.NA  # <-- incorrectly casts to object, lets cast back and ignore that for now
df = df.astype("Int64")

res = df.mean()

>>> res
0     0.0
1    <NA>
2   -0.25
dtype: object
```

Trivial to fix with 2D EAs, not sure how/if to fix it without.