chore(deps): update rig-core requirement from 0.15.1 to 0.21.0

Updates the requirements on [rig-core](https://github.com/0xPlaygrounds/rig) to permit the latest version.
- [Release notes](https://github.com/0xPlaygrounds/rig/releases)
- [Commits](0xPlaygrounds/rig@rig-core-v0.15.1...rig-core-v0.21.0)

---
updated-dependencies:
- dependency-name: rig-core
  dependency-version: 0.21.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

examples/rig-integration/Cargo.toml

@@ -13,7 +13,7 @@ readme = { workspace = true }
 publish = false
 
 [dependencies]
-rig-core = "0.15.1"
+rig-core = "0.21.0"
 tokio = { version = "1", features = ["full"] }
 rmcp = { workspace = true, features = [
     "client",