Update egui requirement from 0.19.0 to 0.21.0 in /serial_term

Updates the requirements on [egui](https://github.com/emilk/egui) to permit the latest version.
- [Release notes](https://github.com/emilk/egui/releases)
- [Changelog](https://github.com/emilk/egui/blob/master/CHANGELOG.md)
- [Commits](emilk/egui@0.19.0...0.21.0)

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

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

Author: dependabot[bot]

serial_term/Cargo.toml

@@ -7,5 +7,5 @@ edition = "2021"
 
 [dependencies]
 eframe = "0.21.2"
-egui = "0.19.0"
+egui = "0.21.0"
 serialport = "4.2.0"