Use `ignored` color from theme for items ignored by Git (#10038)

This PR updates the color of the label used for Git-aware items to use
the `ignored` color from the theme when the item is ignored by Git.

The built-in themes have had their `ignored` color updated to match
`text.disabled`, as the existing `ignored` color did not sufficiently
differentiate from non-ignored items.

Fixes #9976.

Release Notes:

- Updated items in the project panel to use the `ignored` color from the
theme when they are ignored by Git
([#9976](https://github.com/zed-industries/zed/issues/9976)).