Update version to 1.8.0 (#518)

Author: maciejdudko

Directory.Build.props

@@ -17,7 +17,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <Version>1.7.0</Version>
+    <Version>1.8.0</Version>
     <!--
       .NET does not allow the above version format for AssemblyVersion, and this
       is the version used in gRPC headers. The format is
@@ -32,7 +32,7 @@
       and 0.2.0 then is 0.2.0.5. But if there is no prerelease version, just
       leave revision off.
     -->
-    <AssemblyVersion>1.7.0</AssemblyVersion>
+    <AssemblyVersion>1.8.0</AssemblyVersion>
   </PropertyGroup>
 
   <ItemGroup>