chore(deps): Bump Portable.System.DateTimeOnly from 7.0.1 to 8.0.0

Bumps [Portable.System.DateTimeOnly](https://github.com/OlegRa/System.DateTimeOnly) from 7.0.1 to 8.0.0.
- [Release notes](https://github.com/OlegRa/System.DateTimeOnly/releases)
- [Commits](OlegRa/System.DateTimeOnly@v7.0.1...v8.0.0)

---
updated-dependencies:
- dependency-name: Portable.System.DateTimeOnly
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

test/Riok.Mapperly.IntegrationTests/Riok.Mapperly.IntegrationTests.csproj

@@ -43,7 +43,7 @@
   <!-- cannot use source generated polyfills since they require language version 11 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'net48'">
     <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
-    <PackageReference Include="Portable.System.DateTimeOnly" Version="7.0.1" />
+    <PackageReference Include="Portable.System.DateTimeOnly" Version="8.0.0" />
     <PackageReference Include="IsExternalInit" Version="1.0.3">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>