[Text Analytics] Prepare for 5.1.0-beta.5 release (Azure#14247)

Now that [core v2](https://www.npmjs.com/package/@azure/core-rest-pipeline/v/1.0.0-beta.2) has been released, we can release! This PR updates date in changelog.

Author: deyaaeldeen

sdk/textanalytics/ai-text-analytics/CHANGELOG.md

@@ -1,6 +1,6 @@
 # Release History
 
-## 5.1.0-beta.5 (Unreleased)
+## 5.1.0-beta.5 (2021-03-11)
 
 - We are now targeting the service's v3.1-preview.4 API as the default instead of v3.1-preview.3.
 - [Breaking] Aspects in opinions mining are now called targets and each individual opinion is now called an assessment. The new naming simplifies the naming of different parts of the response.