[Dependabot]: Bump Avalonia and Avalonia.Themes.Simple

Bumps Avalonia from 12.0.0 to 12.1.3
Bumps Avalonia.Themes.Simple from 12.0.0 to 12.1.3

---
updated-dependencies:
- dependency-name: Avalonia
  dependency-version: 12.1.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: Avalonia.Themes.Simple
  dependency-version: 12.1.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

Directory.Build.props

@@ -1,5 +1,5 @@
 <Project>
   <PropertyGroup>
-    <AvaloniaVersion>12.0.0</AvaloniaVersion>
+    <AvaloniaVersion>12.1.3</AvaloniaVersion>
   </PropertyGroup>
 </Project>