Update yew requirement from 0.20.0 to 0.21.0

Updates the requirements on [yew](https://github.com/yewstack/yew) to permit the latest version.
- [Release notes](https://github.com/yewstack/yew/releases)
- [Changelog](https://github.com/yewstack/yew/blob/master/CHANGELOG.md)
- [Commits](yewstack/yew@yew-v0.20.0...yew-v0.21.0)

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

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

bui-demo/frontend_yew/Cargo.toml

@@ -8,7 +8,7 @@ edition = "2018"
 crate-type = ["cdylib", "rlib"]
 
 [dependencies]
-yew = {version="0.20.0", features=["csr"]}
+yew = {version="0.21.0", features=["csr"]}
 wasm-bindgen = "^0.2"
 js-sys = "0.3"
 log = "0.4"