Remove release notes section

Feedback has shown that having similar fields in the release notes
section to the main headings is confusing. This PR simplifies the
sections and removes the release notes section completely.

Changes will be required to the release scripts and the docs to
reflect these changes.

Author: adbridge

.github/pull_request_template.md

@@ -16,7 +16,28 @@ template is automatically parsed.
     the issue is and add the reference. E.g. Fixes #17119).
 
     Any implications for users taking this change.
- -->
+    
+    NOTE: This section is automatically written to release notes for Feature and 
+    Major releases and should contain enough details for a user.
+    
+-->
+
+### Impact of changes <!-- Optional -->
+<!-- 
+    If there are any implications for users taking this change then they must be 
+    provided here. For Major PR types this field is MANDATORY.
+
+    NOTE: This section is automatically written to release notes for Feature and 
+    Major releases and should contain enough details for a user.
+-->
+
+### Migration actions required <!-- Optional -->
+<!-- 
+    This should only be applicable in Major PR types for which this field is MANDATORY.
+
+    NOTE: This section is automatically written to release notes for Feature and 
+    Major releases and should contain enough details for a user.
+-->
 
 ### Documentation <!-- Required -->
 
@@ -57,16 +78,3 @@ template is automatically parsed.
 -->
 
 ----------------------------------------------------------------------------------------------------------------
-### Release Notes <!-- Required for features, deprecations, breaking changes and other major PRs -->
-
-<!--
-    All 3 sections are compulsory for Major PR types. For Feature PRs only the summary section is required.
-    This section is automatically added to release notes. Please fill in each sub-section with sufficient detail for a user.
-    For more information, please see [the contributing guidelines](https://os.mbed.com/docs/mbed-os/latest/contributing/workflow.html#pull-request-types). 
--->
-
-#### Summary of changes
-
-#### Impact of changes
-
-#### Migration actions required