update Pomelo.EntityFrameworkCore.MySql to 8.0.0-beta.2, as 7.0 version does not support EF Core 8.0

Author: adamsitnik

src/HealthChecks.UI.MySql.Storage/HealthChecks.UI.MySql.Storage.csproj

@@ -8,7 +8,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Pomelo.EntityFrameworkCore.MySql" Version="7.0.0 " />
+    <PackageReference Include="Pomelo.EntityFrameworkCore.MySql" Version="8.0.0-beta.2" />
   </ItemGroup>
 
   <ItemGroup>