Pin FluentAssertions to version 7

Because of the license change introduced in fluentassertions/fluentassertions#2943

Author: 0xced

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.3" PrivateAssets="all" />
-    <PackageReference Include="FluentAssertions" Version="7.0.0" />
+    <PackageReference Include="FluentAssertions" Version="[7.0.0,8.0.0)" />
     <PackageReference Include="GitHubActionsTestLogger" Version="2.4.1" PrivateAssets="all" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageReference Include="PublicApiGenerator" Version="11.3.0" />