3.15.0-preview: Adds new version and changelog (#1950)

* Bump the API version to 3.15.0-preview

Author: j82w

Directory.Build.props

@@ -2,7 +2,7 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
     <ClientOfficialVersion>3.14.0</ClientOfficialVersion>
-    <ClientPreviewVersion>3.14.0</ClientPreviewVersion>
+    <ClientPreviewVersion>3.15.0</ClientPreviewVersion>
     <DirectVersion>3.14.1</DirectVersion>
     <EncryptionVersion>1.0.0-preview6</EncryptionVersion>
     <HybridRowVersion>1.1.0-preview1</HybridRowVersion>