Bump SixLabors.ImageSharp in /core/Piranha.ImageSharp

Bumps [SixLabors.ImageSharp](https://github.com/SixLabors/ImageSharp) from 2.1.7 to 2.1.8.
- [Release notes](https://github.com/SixLabors/ImageSharp/releases)
- [Commits](SixLabors/ImageSharp@v2.1.7...v2.1.8)

---
updated-dependencies:
- dependency-name: SixLabors.ImageSharp
  dependency-type: direct:production
...

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

core/Piranha.ImageSharp/Piranha.ImageSharp.csproj

@@ -1,17 +1,17 @@
-<Project Sdk="Microsoft.NET.Sdk">
-
-  <PropertyGroup>
-    <NoWarn>$(NoWarn);1591</NoWarn>
-    <Description>Package for processing uploaded images using ImageSharp</Description>
-  </PropertyGroup>
-
-  <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
-    <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
-  </PropertyGroup>
-
-  <ItemGroup>
-    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.7" />
-    <ProjectReference Include="..\Piranha\Piranha.csproj" />
-  </ItemGroup>
-
-</Project>
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <NoWarn>$(NoWarn);1591</NoWarn>
+    <Description>Package for processing uploaded images using ImageSharp</Description>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">
+    <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.8" />
+    <ProjectReference Include="..\Piranha\Piranha.csproj" />
+  </ItemGroup>
+
+</Project>