Update strum_macros requirement from 0.20 to 0.21 (#1901)

Updates the requirements on [strum_macros](https://github.com/Peternator7/strum) to permit the latest version.
- [Release notes](https://github.com/Peternator7/strum/releases)
- [Changelog](https://github.com/Peternator7/strum/blob/master/CHANGELOG.md)
- [Commits](https://github.com/Peternator7/strum/commits)

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

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

Author: dependabot[bot]

examples/todomvc/Cargo.toml

@@ -7,7 +7,7 @@ license = "MIT OR Apache-2.0"
 
 [dependencies]
 strum = "0.21"
-strum_macros = "0.20"
+strum_macros = "0.21"
 serde = "1"
 serde_derive = "1"
 yew = { path = "../../packages/yew" }