Fix gitignore with Resource.designer.cs/Resource.Designer.cs

Default value for <AndroidResgenFile> is Resource.designer.cs
https://learn.microsoft.com/en-us/xamarin/android/deploy-test/building-apps/build-properties#androidresgenfile

Two projects were using <AndroidResgenFile>Resources\Resource.Designer.cs</AndroidResgenFile>
All other projects use <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
Difference is one has uppercase D, the other lowercase d

.gitignore already has a rule for 'Resource.designer.cs' in it.

The two projects that were with Resource.Designer.cs needed either a specific line in gitignore
or weren't actually ignored.

Selected fix here is to rename the file with a lowercase 'd' instead of uppercase.
This permits to remove one line from .gitignore and keep the other file really ignored

src/TwofishCipher/TwofishCipher.csproj

@@ -10,7 +10,7 @@
     <RootNamespace>TwofishCipher</RootNamespace>
     <AssemblyName>TwofishCipher</AssemblyName>
     <FileAlignment>512</FileAlignment>
-    <AndroidResgenFile>Resources\Resource.Designer.cs</AndroidResgenFile>
+    <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
     <GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
     <AndroidUseLatestPlatformSdk>false</AndroidUseLatestPlatformSdk>
     <TargetFrameworkVersion>v8.0</TargetFrameworkVersion>
@@ -54,7 +54,7 @@
     <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
-    <Compile Include="Resources\Resource.Designer.cs" />
+    <Compile Include="Resources\Resource.designer.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Twofish.cs" />
     <Compile Include="TwofishBase.cs" />
@@ -81,4 +81,4 @@
   <Target Name="AfterBuild">
   </Target>
   -->
-</Project>
+</Project>