[ai-form-recognizer] Prepare 4.0.0 beta.6 release (Azure#22856)

* Fix polygon in document line.

* Set release date.

* Fix model ID in prebuilt-vaccinationCard sample

* Update README.

Author: witemple-msft

sdk/formrecognizer/ai-form-recognizer/CHANGELOG.md

@@ -1,6 +1,6 @@
 # Release History
 
-## 4.0.0-beta.6 (Unreleased)
+## 4.0.0-beta.6 (2022-08-09)
 
 ### Features Added
 
@@ -15,8 +15,6 @@
 - Changed the suffix `-Info` for the methods and interfaces such as `DocumentModelAdministrationClient#getResourceInfo` and `DocumentModelInfo` to `-Details`.
 - Array properties of `AnalyzeResult`, such as `documents`, `languages`, and `pages` are now optional. If the value is `undefined`, then the model does not support the given feature. Previously, we returned an empty array, even if the model didn't support the feature.
 
-### Bugs Fixed
-
 ### Other Changes
 
 - Strongly-typed analysis functionality now checks that the `DocumentAnalysisClient`'s API version now matches the assumed API version of the `DocumentModel` exactly. This ensures that a strongly-typed model can only be used with the API version it was created for, so that future changes cannot violate the model's schema.