Update indexmap to 2.6.0

Author: VZout

crates/rustc_codegen_spirv/Cargo.toml

@@ -49,7 +49,7 @@ regex = { version = "1", features = ["perf"] }
 # Normal dependencies.
 ar = "0.9.0"
 either = "1.8.0"
-indexmap = "1.6.0"
+indexmap = "2.6.0"
 rspirv = "0.12"
 rustc_codegen_spirv-types.workspace = true
 rustc-demangle = "0.1.21"