Update indexmap requirement from 1 to 2

Updates the requirements on [indexmap](https://github.com/bluss/indexmap) to permit the latest version.
- [Changelog](https://github.com/bluss/indexmap/blob/master/RELEASES.md)
- [Commits](indexmap-rs/indexmap@1.0.0...2.0.0)

---
updated-dependencies:
- dependency-name: indexmap
  dependency-type: direct:production
...

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

generator/Cargo.toml

@@ -12,5 +12,5 @@ quote = "1.0"
 syn = { version = "2.0", default-features = false, features = ["derive", "printing"] }
 proc-macro2 = "1.0"
 heck = "0.4"
-indexmap = "1"
+indexmap = "2"
 regex = "1.2.1"