Update introduced milestone for namespace metadata fields which missed 18.6.0

The milestone version in the namespace metadata types is incorrect since the MR was merged after applying the missed:18.6 label. The 18.6.0 instances will not have these fields available.

This was discovered while testing a query backwards compatible with 18.3 instances attempting to use one of these fields against a self-managed instance running on 18.6.0-ee.

Related note: !211059 (comment 2915123287)

Expected result metadata to be present:

query lsp_namespaceOrUser($namespace: ID!) {
  group(fullPath: $namespace) {
    id
    fullPath
    parent {
      id
    }
  }
  namespace(fullPath: $namespace) {
    id
    name
    fullPath
    path
    metadata @gl_introduced(version: "18.6.0") {
      groupId
    }
  }
}

Current Behaviour:

{
  "errors": [
    {
      "graphQLErrors": [
        {
          "message": "Field 'metadata' doesn't exist on type 'Namespace'",
          "locations": [
            {
              "line": 14,
              "column": 5
            }
          ],
          "path": [
            "query lsp_namespaceOrUser",
            "namespace",
            "metadata"
          ],
          "extensions": {
            "code": "undefinedField",
            "typeName": "Namespace",
            "fieldName": "metadata"
          }
        }
      ],
      "clientErrors": [],
      "networkError": null,
      "message": "Field 'metadata' doesn't exist on type 'Namespace'",
      "stack": "...TRUNCATED..."
    }
  ]
}```

Proposed solution: Swap occurrences of `18.6` for `18.7` based on the modifications introduced by https://gitlab.com/gitlab-org/gitlab/-/merge_requests/211059.