upgrade guide (#3100)

Author: maastha

docs/guides/1.28.0-upgrade-guide.md

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+---
+page_title: "Upgrade Guide 1.28.0"
+---
+
+# MongoDB Atlas Provider 1.28.0: Upgrade and Information Guide
+
+The Terraform MongoDB Atlas Provider version 1.28.0 has a number of new and exciting features.
+
+## New Resources, Data Sources, and Features
+
+- You can now use `mongodbatlas_encryption_at_rest` resource & data source to manage encryption of your data at rest in Atlas with the customer-managed keys (CMK) that you create, own, and manage in your AWS KMS by adding another layer of security and configuring all traffic to your AWS KMS to use AWS PrivateLink via `aws_kms_config.requirePrivateNetworking` attribute and `mongodbatlas_encryption_at_rest_private_endpoint` resource. For more details, see [mongodbatlas_encryption_at_rest](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/encryption_at_rest) resource documentation.
+
+
+## Helpful Links
+
+* [Report bugs](https://github.com/mongodb/terraform-provider-mongodbatlas/issues)
+
+* [Request Features](https://feedback.mongodb.com/forums/924145-atlas?category_id=370723)
+
+* [Contact Support](https://docs.atlas.mongodb.com/support/) covered by MongoDB Atlas support plans, Developer and above.