[Doc] mark databricks_volume and databricks_credential as GA (#4524)

## Changes
- UC volume & service credential are both GA

## Tests
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How is this tested? Please see the checklist below and also describe any
other relevant tests
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- [x] relevant change in `docs/` folder

NO_CHANGELOG=true

Author: nkvuong

docs/resources/credential.md

@@ -5,8 +5,6 @@ subcategory: "Unity Catalog"
 
 -> This resource can only be used with a workspace-level provider.
 
--> This feature is in [Public Preview](https://docs.databricks.com/release-notes/release-types.html).
-
 A credential represents an authentication and authorization mechanism for accessing services on your cloud tenant. Each credential is subject to Unity Catalog access-control policies that control which users and groups can access the credential.
 
 The type of credential to be created is determined by the `purpose` field, which should be either `SERVICE` or `STORAGE`.

docs/resources/volume.md

@@ -3,8 +3,6 @@ subcategory: "Unity Catalog"
 ---
 # databricks_volume (Resource)
 
--> This feature is in [Public Preview](https://docs.databricks.com/release-notes/release-types.html).
-
 -> This resource can only be used with a workspace-level provider!
 
 Volumes are Unity Catalog objects representing a logical volume of storage in a cloud object storage location. Volumes provide capabilities for accessing, storing, governing, and organizing files. While tables provide governance over tabular datasets, volumes add governance over non-tabular datasets. You can use volumes to store and access files in any format, including structured, semi-structured, and unstructured data.