Update indexmap requirement from 1 to 2

Updates the requirements on [indexmap](https://github.com/indexmap-rs/indexmap) to permit the latest version.
- [Changelog](https://github.com/indexmap-rs/indexmap/blob/master/RELEASES.md)
- [Commits](indexmap-rs/indexmap@1.0.0...2.2.5)

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

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

pingora-cache/Cargo.toml

@@ -24,7 +24,7 @@ pingora-http = { version = "0.1.0", path = "../pingora-http" }
 pingora-lru = { version = "0.1.0", path = "../pingora-lru" }
 pingora-timeout = { version = "0.1.0", path = "../pingora-timeout" }
 http = { workspace = true }
-indexmap = "1"
+indexmap = "2"
 once_cell = { workspace = true }
 regex = "1"
 blake2 = "0.10"