3.26.0-preview: Adds changelog and contract file (#3054)

3.26.0-preview release to unblock a new encryption SDK release.

Author: j82w

Directory.Build.props

@@ -2,7 +2,7 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 	<PropertyGroup>
 		<ClientOfficialVersion>3.25.0</ClientOfficialVersion>
-		<ClientPreviewVersion>3.25.0</ClientPreviewVersion>
+		<ClientPreviewVersion>3.26.0</ClientPreviewVersion>
 		<ClientPreviewSuffixVersion>preview</ClientPreviewSuffixVersion>
 		<DirectVersion>3.24.1</DirectVersion>
 		<EncryptionVersion>1.0.0-previewV19</EncryptionVersion>