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_plugin_formats/Cargo.toml

@@ -12,7 +12,7 @@ version = "0.82.1"
 nu-plugin = { path = "../nu-plugin", version = "0.82.1" }
 nu-protocol = { path = "../nu-protocol", version = "0.82.1", features = ["plugin"] }
 
-indexmap = { version = "1.7", features = ["serde-1"] }
+indexmap = "2.0"
 eml-parser = "0.1"
 ical = "0.8"
 rust-ini = "0.19.0"