chore: auto-update metadata coverage in METADATA_SUPPORT.md [no ci]

Author: svc-cli-bot

METADATA_SUPPORT.md

@@ -4,7 +4,7 @@ This list compares metadata types found in Salesforce v64 with the [metadata reg
 
 This repository is used by both the Salesforce CLIs and Salesforce's VSCode Extensions.
 
-Currently, there are 655/689 supported metadata types.
+Currently, there are 656/689 supported metadata types.
 For status on any existing gaps, please search or file an issue in the [Salesforce CLI issues only repo](https://github.com/forcedotcom/cli/issues).
 To contribute a new metadata type, please see the [Contributing Metadata Types to the Registry](./contributing/metadata.md)
 
@@ -657,7 +657,7 @@ To contribute a new metadata type, please see the [Contributing Metadata Types t
 |UIObjectRelationConfig|✅||
 |UiFormatSpecificationSet|✅||
 |UiPlugin|✅||
-|UiPreviewMessageTabDef|❌|Not supported, but support could be added|
+|UiPreviewMessageTabDef|✅||
 |UserAccessPolicy|✅||
 |UserAuthCertificate|✅||
 |UserCriteria|✅||