doc: Adds v1.27.0 upgrade guide (#3094)

* 1.27 upgrade guide

* pr comments

* Update docs/guides/1.27.0-upgrade-guide.md

Co-authored-by: Jeff Vincent <[email protected]>

---------

Co-authored-by: Jeff Vincent <[email protected]>

Author: oarbusi

docs/guides/1.27.0-upgrade-guide.md

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+---
+page_title: "Upgrade Guide 1.27.0"
+---
+
+# MongoDB Atlas Provider 1.27.0: Upgrade and Information Guide
+
+The Terraform MongoDB Atlas Provider version 1.27.0 has a number of new and exciting features.
+
+## New Resources, Data Sources, and Features
+
+- You can now manage Flex Clusters using the `mongodbatlas_advanced_cluster` resource and corresponding data sources. To learn more, please review the [mongodbatlas_advanced_cluster](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/advanced_cluster) resource documentation.
+- [Atlas Flex tier](https://www.mongodb.com/blog/post/dynamic-workloads-predictable-costs-mongodb-atlas-flex-tier) is now Generally Available (GA) and `mongodbatlas_flex_cluster` resource and data sources are no longer in Public Preview. 
+- You can now retrieve Flex cluster snapshots using `mongodbatlas_flex_snapshot` data sources. To learn more, please review the [mongodbatlas_flex_snapshot](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/data-sources/flex_snapshot) data source documentation.
+- You can now retrieve Flex cluster snapshots using `mongodbatlas_flex_restore_job` data sources. To learn more, please review the [mongodbatlas_flex_restore_job](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/data-sources/flex_restore_job) data source documentation.
+- You can now update `mongodbatlas_global_cluster_config` resource. For more details, see [#3060](https://github.com/mongodb/terraform-provider-mongodbatlas/pull/3060)
+
+## Deprecations and Removals
+
+- `mongodbatlas_serverless_instance` resouce and corresponding data sources have been deprecated. For more details, see [Migration Guide: Transition out of Serverless Instances and Shared-tier clusters](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/serverless-shared-migration-guide)
+- `mongodbatlas_shared_tier_restore_job` data sources have been deprecated. For more details, see [Migration Guide: Transition out of Serverless Instances and Shared-tier clusters](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/serverless-shared-migration-guide)
+- `mongodbatlas_shared_tier_snapshot` data sources have been deprecated. For more details, see [Migration Guide: Transition out of Serverless Instances and Shared-tier clusters](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/serverless-shared-migration-guide)
+- Instance size values `M2` and `M5` for the attribute `provider_instance_size_name` in `mongodbatlas_cluster` have been deprecated in favor of the new Flex tier in `mongodbatlas_advanced_cluster` resource. To learn more, please review the [mongodbatlas_advanced_cluster](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/advanced_cluster#example-flex-cluster) resource documentation.
+- Instance size values `M2` and `M5` for the attribute `instance_size` inside of `analytics_specs`, `electable_specs` and `read_only_specs` in the resouce `mongodbatlas_advanced_cluster` have been deprecated in favor of the new Flex tier in `mongodbatlas_advanced_cluster` resource. To learn more, please review the [mongodbatlas_advanced_cluster](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/advanced_cluster#example-flex-cluster) 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.