Dependabot: Remove the now-unsupported reviewers field (SchemaStore#4905)

On 2025-04-29, GitHub announced that `reviewers` would be removed in the future:

https://github.blog/changelog/2025-04-29-dependabot-reviewers-configuration-option-being-replaced-by-code-owners/

On 2025-08-08, GitHub announced that `reviewers` was officially removed:

https://github.blog/changelog/2025-08-08-dependabot-reviewers-configuration-option-is-replaced-by-code-owners/

This commit removes all of the negative `reviewers` tests
and then moves the positive test to the negative test directory
with the name `reviewers-no-longer-valid-2025-08-08.json`.

Author: kurtmckee

src/negative_test/dependabot-2.0/reviewers-duplicate-values.json

@@ -978,16 +978,6 @@
             }
           ]
         },
-        "reviewers": {
-          "type": "array",
-          "items": {
-            "type": "string",
-            "minLength": 1
-          },
-          "description": "Specify individual reviewers or teams of reviewers for all pull requests raised for a package manager. You must use the full team name, including the organization, as if you were @mentioning the team.",
-          "minItems": 1,
-          "uniqueItems": true
-        },
         "schedule": {
           "description": "Schedule preferences",
           "type": "object",