Bump indexmap from 2.2.2 to 2.6.0

Bumps [indexmap](https://github.com/indexmap-rs/indexmap) from 2.2.2 to 2.6.0.
- [Changelog](https://github.com/indexmap-rs/indexmap/blob/master/RELEASES.md)
- [Commits](indexmap-rs/indexmap@2.2.2...2.6.0)

---
updated-dependencies:
- dependency-name: indexmap
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -36,7 +36,7 @@ yaml-rust2 = { version = "0.8", optional = true }
 rust-ini = { version = "0.20", optional = true }
 ron = { version = "0.8", optional = true }
 json5_rs = { version = "0.4", optional = true, package = "json5" }
-indexmap = { version = "2.2", features = ["serde"], optional = true }
+indexmap = { version = "2.6", features = ["serde"], optional = true }
 convert_case = { version = "0.6", optional = true }
 pathdiff = "0.2"