chore: Adds upgrade guide for 1.21.0 (#2658)

* upgrade guide for 1.21.0

* Update docs/guides/1.21.0-upgrade-guide.md

* add redact and modules info

---------

Co-authored-by: Leo Antoli <[email protected]>

Author: oarbusi

docs/guides/1.21.0-upgrade-guide.md

@@ -0,0 +1,31 @@
+---
+page_title: "Upgrade Guide 1.21.0"
+---
+
+# MongoDB Atlas Provider 1.21.0: Upgrade and Information Guide
+
+The Terraform MongoDB Atlas Provider version 1.21.0 has a number of new and exciting features.
+
+## New Resources, Data Sources, and Features
+
+- You can now manage Resource Policies with the new `mongodbatlas_resource_policy` resource and corresponding data sources. The feature is available as a preview feature. To learn more, please review `mongodbatlas_resource_policy` [resource documentation](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/resource_policy).
+
+- `custom_zone_mapping_zone_id` attribute has been added to `mongodbatlas_global_cluster_config` resource and data source. To learn more, please review the resource [documentation](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/global_cluster_config#custom_zone_mapping_zone_id).
+
+- `redact_client_log_data` attribute has been added to `mongodbatlas_advanced_cluster`, `mongodbatlas_cluster` and corresponding data sources.
+
+## Deprecations and removals
+
+- `custom_zone_mapping` attribute has been deprecated in `mongodbatlas_global_cluster_config` resource and data source in favor of the new `custom_zone_mapping_zone_id` [attribute](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/global_cluster_config#custom_zone_mapping_zone_id).
+
+## Terraform MongoDB Atlas modules
+
+You can now leverage our [Terraform Modules](https://registry.terraform.io/namespaces/terraform-mongodbatlas-modules) to easily get started with MongoDB Atlas and critical features like [Push-based log export](https://registry.terraform.io/modules/terraform-mongodbatlas-modules/push-based-log-export/mongodbatlas/latest), [Private Endpoints](https://registry.terraform.io/modules/terraform-mongodbatlas-modules/private-endpoint/mongodbatlas/latest), etc.
+
+## 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.