Match type hint with ResidualUnit and Convolution (#6538)

### Description

I noticed the type hint for `dropout` in `ConvDenseBlock` did not match
the ones further down the file, when `dropout` is passed on to
`ResidualUnit` or `Convolution`.
This pull request changes this type hint to match the downstream ones.

I don't think this will cause issues with the unit tests, but they're
currently running while I prepare this pull request.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [x] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.

Signed-off-by: Mathijs de Boer <m.deboer-41@umcutrecht.nl>
Co-authored-by: Mathijs de Boer <m.deboer-41@umcutrecht.nl>

Author: MathijsdeBoer

monai/networks/blocks/denseblock.py

@@ -74,7 +74,7 @@ def __init__(
         adn_ordering: str = "NDA",
         act: tuple | str | None = Act.PRELU,
         norm: tuple | str | None = Norm.INSTANCE,
-        dropout: int | None = None,
+        dropout: tuple | str | float | None = None,
         bias: bool = True,
     ):
         self.spatial_dims = spatial_dims