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>

Author: dependabot[bot]

gdnative-core/Cargo.toml

@@ -26,7 +26,7 @@ approx = "0.5"
 atomic-take = "1"
 bitflags = "1"
 glam = "0.24"
-indexmap = "1"
+indexmap = "2"
 inventory = { version = "0.3", optional = true }
 libc = "0.2"
 once_cell = "1"