Hoist indexmap to workspace level (#12901)

This PR hoists `indexmap` up to a workspace dependency.

Release Notes:

- N/A

crates/theme/Cargo.toml

@@ -24,7 +24,7 @@ derive_more.workspace = true
 fs.workspace = true
 futures.workspace = true
 gpui.workspace = true
-indexmap = { version = "1.6.2", features = ["serde"] }
+indexmap.workspace = true
 palette = { workspace = true, default-features = false, features = ["std"] }
 parking_lot.workspace = true
 refineable.workspace = true