gpui: Bump taffy to 0.4.3 again (#11655)

We reverted bump to taffy 0.4.3 following an issue spotted by
@maxdeviant where chat text input was not being rendered correctly:

![image](https://github.com/zed-industries/zed/assets/24362066/9d7e6444-47b1-4ac2-808f-bf10404377c0)
This was an issue with the previous attempt to upgrade to taffy 0.4.0 as
well. We bail early in `compute_auto_height_layout` due to a missing
width:
df190ea846/crates/editor/src/element.rs (L5266)
The same issue is visible in story for auto-height editor (or rather,
the breakage is visible - the editor simply does not render at all
there).

I tracked down the breakage to
https://github.com/DioxusLabs/taffy/pull/573 ; it looks like it
specifically affects editors with auto-height. In taffy <0.4 which we
were using previously, we'd eventually get a proper width for
auto-height EditorElement after having initially computed the size. With
taffy 0.4 however (and specifically that PR mentioned earlier), we're
getting `Size::NONE` in layout phase [^1].
I've noticed though that even with taffy <0.3, the
`known_dimensions.width` was always equal to `available_space.width` in
layout phase. Hence, I went with falling back to `available_space.width`
when it is a definite value and we don't have a
`known_dimensions.width`.
Done this way, both chat input and auto-height story render correctly.
/cc @as-cii 
Related:
https://github.com/zed-industries/zed/pull/11606
https://github.com/zed-industries/zed/pull/11622
https://github.com/zed-industries/zed/pull/7868
https://github.com/zed-industries/zed/pull/7896

[^1]: This could possibly be related to change in what gets passed in
https://github.com/DioxusLabs/taffy/pull/573/files#diff-60c916e9b0c507925f032cecdde6ae163e41b84b8e4bc0a6c04f7d846b0aad9eR133
, though I'm not sure if editor is a leaf node in this case

Release Notes:

- N/A

crates/gpui/Cargo.toml

@@ -61,7 +61,7 @@ slotmap = "1.0.6"
 smallvec.workspace = true
 smol.workspace = true
 sum_tree.workspace = true
-taffy = { git = "https://github.com/DioxusLabs/taffy", rev = "1876f72bee5e376023eaa518aa7b8a34c769bd1b" }
+taffy = "0.4.3"
 thiserror.workspace = true
 time.workspace = true
 util.workspace = true