Bump indexmap to 2.0 (#9643)

# Description
Apart from `polars` (only used with `--features dataframe`) and the
dev-dependencies our deps use `indexmap 2.0`.
Thus the default or `extra` `cargo build` will reduce deps.
This also will help deduplicating `hashbrown` and `ahash`.

For #8060

- Bump `indexmap` to 2.0
- Remove unneeded `serde` feature from `indexmap`

# User-Facing Changes
None

crates/nu-command/Cargo.toml

@@ -49,7 +49,7 @@ filesize = "0.2"
 filetime = "0.2"
 fs_extra = "1.3"
 htmlescape = "0.3"
-indexmap = { version = "1.7", features = ["serde-1"] }
+indexmap = "2.0"
 indicatif = "0.17"
 is-root = "0.1"
 itertools = "0.10"