Update downcast-rs requirement from 1.1.1 to 2.0.0

Updates the requirements on [downcast-rs](https://github.com/marcianx/downcast-rs) to permit the latest version.
- [Changelog](https://github.com/marcianx/downcast-rs/blob/master/CHANGELOG.md)
- [Commits](marcianx/downcast-rs@v1.1.1...v2.0.0)

---
updated-dependencies:
- dependency-name: downcast-rs
  dependency-type: direct:production
...

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

client/Cargo.toml

@@ -30,7 +30,7 @@ whoami = "1.2.1"
 serde = { version = "1.0.104", features = ["derive", "rc"] }
 toml = { workspace = true }
 fxhash = "0.2.1"
-downcast-rs = "1.1.1"
+downcast-rs = "2.0.0"
 quinn = { workspace = true }
 futures-util = "0.3.1"
 webpki = "0.22.0"