build(deps): bump rhai from 1.20.1 to 1.21.0 in /native/rhai_rustler (#256)

Bumps [rhai](https://github.com/rhaiscript/rhai) from 1.20.1 to 1.21.0.
- [Release notes](https://github.com/rhaiscript/rhai/releases)
- [Changelog](https://github.com/rhaiscript/rhai/blob/main/CHANGELOG.md)
- [Commits](rhaiscript/rhai@v1.20.1...v1.21.0)

---
updated-dependencies:
- dependency-name: rhai
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

native/rhai_rustler/Cargo.lock

@@ -10,7 +10,7 @@ path = "src/lib.rs"
 crate-type = ["cdylib"]
 
 [dependencies]
-rhai = { version = "=1.20.1", features = ["sync"] }
+rhai = { version = "=1.21.0", features = ["sync"] }
 rhai-dylib = { version = "0.4.0", features = ["sync"] }
 thiserror = "2.0.11"
 rustler = "0.36.0"