Prepare for Azure.Core 1.8.0 release (Azure#17803)

Author: pakrym

sdk/core/Azure.Core/CHANGELOG.md

@@ -1,6 +1,6 @@
 # Release History
 
-## 1.8.0-beta.1 (Unreleased)
+## 1.8.0 (2020-01-06)
 
 ### Added
 - `AzureSasCredential` and its respective policy.

sdk/core/Azure.Core/src/Azure.Core.csproj

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <Description>This is the implementation of the Azure Client Pipeline</Description>
     <AssemblyTitle>Microsoft Azure Client Pipeline</AssemblyTitle>
-    <Version>1.8.0-beta.1</Version>
+    <Version>1.8.0</Version>
     <ApiCompatVersion>1.7.0</ApiCompatVersion>
     <PackageTags>Microsoft Azure Client Pipeline</PackageTags>
     <Nullable>enable</Nullable>