doc: Upgrade guide for v1.31.0 (#3205)

* upgrade guide

* Update docs/guides/1.31.0-upgrade-guide.md

Co-authored-by: maastha <[email protected]>

* plugin reference

* doc changes.

* Update docs/guides/1.31.0-upgrade-guide.md

Co-authored-by: Colm Quinn <[email protected]>

* address comment.

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Co-authored-by: Leo Antoli <[email protected]>
Co-authored-by: maastha <[email protected]>
Co-authored-by: Marco Suma <[email protected]>
Co-authored-by: Colm Quinn <[email protected]>

Author: 5 people

docs/guides/1.31.0-upgrade-guide.md

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+---
+page_title: "Upgrade Guide 1.31.0"
+---
+
+# MongoDB Atlas Provider 1.31.0: Upgrade and Information Guide
+
+The Terraform MongoDB Atlas Provider version 1.31.0 has a number of new and exciting features.
+
+## New Resources, Data Sources, and Features
+
+- You can now optionally skip creating the default set of alerts for new organizations, using the `skip_default_alerts_settings` attribute  in the `mongodbatlas_organization` resource & data sources. To learn more, see the [mongodbatlas_organization](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/organization) resource documentation.
+
+- You can now use the new [Atlas CLI plugin](https://github.com/mongodb-labs/atlas-cli-plugin-terraform) to help with the migration from `mongodbatlas_cluster` to `mongodbatlas_advanced_cluster`. For detailed instructions, see the [Migration Guide: Cluster to Advanced Cluster](cluster-to-advanced-cluster-migration-guide).
+
+## 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.