Fixed [NavigationRailDestination]'s label opacity while disabled not being coherent with the icon (#132345)

Fixing the opacity of the NavigationRailDestination widget label while
it is disabled, right now it doesn't get affected by the disabled
attribute, which doesn't match the icon that gets affected

* https://github.com/flutter/flutter/issues/132344

I believe this PR should be marked as [test-exempt]

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.