Support multi-targeting and reformat exception classes

Updated the project file to support multi-targeting for both
.NET Framework 4.6.2 and .NET 8.0 by replacing `<TargetFramework>`
with `<TargetFrameworks>`.

Reformatted `AuthenticationFailedException` and
`NotFoundException` classes to use braces `{ }` for namespaces
and constructors for consistency. No functional changes were made
to the exception classes.

src/DigitalData.MessagingService.Domain/DigitalData.MessagingService.Domain.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+	<TargetFrameworks>net462;net8.0</TargetFrameworks>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>

src/DigitalData.MessagingService.Domain/Exceptions/AuthenticationFailedException.cs

@@ -1,17 +1,16 @@
-namespace DigitalData.MessagingService.Domain.Exceptions;
-
-/// <summary>
-/// Exception thrown when OAuth2 authentication fails.
-/// </summary>
-public class AuthenticationFailedException : Exception
+namespace DigitalData.MessagingService.Domain.Exceptions
 {
-    public AuthenticationFailedException(string message)
-        : base(message)
+    /// <summary>
+    /// Exception thrown when OAuth2 authentication fails.
+    /// </summary>
+    public class AuthenticationFailedException : Exception
     {
-    }
+        public AuthenticationFailedException(string message) : base(message)
+        {
+        }
 
-    public AuthenticationFailedException(string message, Exception innerException)
-        : base(message, innerException)
-    {
+        public AuthenticationFailedException(string message, Exception innerException) : base(message, innerException)
+        {
+        }
     }
-}
+}

src/DigitalData.MessagingService.Domain/Exceptions/NotFoundException.cs

@@ -1,22 +1,21 @@
-namespace DigitalData.MessagingService.Domain.Exceptions;
-
-/// <summary>
-/// Exception thrown when a requested entity is not found.
-/// </summary>
-public class NotFoundException : Exception
+namespace DigitalData.MessagingService.Domain.Exceptions
 {
-    public NotFoundException(string entityName, object key)
-        : base($"{entityName} with key '{key}' was not found.")
+    /// <summary>
+    /// Exception thrown when a requested entity is not found.
+    /// </summary>
+    public class NotFoundException : Exception
     {
-    }
+        public NotFoundException(string entityName, object key) : base($"{entityName} with key '{key}' was not found.")
+        {
+        }
 
-    public NotFoundException(string message)
-        : base(message)
-    {
-    }
+        public NotFoundException(string message) : base(message)
+        {
+        }
 
-    public NotFoundException(string message, Exception innerException)
-        : base(message, innerException)
-    {
+        public NotFoundException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
     }
-}
+}