[AzureMonitorExporter] prep release 1.4.0 beta2 (Azure#45906)

* disable tests in net6 and MacOS

* prep release

* update date

Author: Timothy Mothra

sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/CHANGELOG.md

@@ -1,10 +1,6 @@
 # Release History
 
-## 1.4.0-beta.2 (Unreleased)
-
-### Features Added
-
-### Breaking Changes
+## 1.4.0-beta.2 (2024-10-11)
 
 ### Bugs Fixed
 
@@ -13,8 +9,6 @@
 * Fixed an issue where unmapped attributes were dropped from telemetry.
   ([#45909](https://github.com/Azure/azure-sdk-for-net/pull/45909))
 
-### Other Changes
-
 ## 1.4.0-beta.1 (2024-07-12)
 
 ### Bugs Fixed