[SalesForce] Method PATCH is not defined in RFC 2068 and is not supported by the Servlet API

Only the Metadata and FullName fields can be updated. Description tag should be included in Metadata as below. Also, include other required Metadata fields and its values. Below request successfully updated the validation rule using Workbench PATCH method.

Request Body:

{
    "Metadata": {
        "description": "Updated via WorkBench",
        "active": true,
        "errorConditionFormula": "NOT(ISNULL(TestField__c))",
        "errorMessage": "Test message"
    }
}

A successful update is a 204 response. That status code means "your operation was successful, and there is no response body." If there were an error, you'd get a 4xx or 5xx status code with a response describing the error. If you want a fresh copy of the data, you will need to retrieve it somehow, such as using a GET call, or using a composite REST resource to patch and retrieve in one call.