Add name field for org api (#21270)

related #21205

The field `UserName` is not really usefull for an organization.
This adds a second `Name` field.

The [GitHub API](https://docs.github.com/en/rest/orgs/orgs#get-an-organization) uses `name` too. `UserName` should be deprecated then.

tests/integration/api_admin_org_test.go

@@ -37,7 +37,7 @@ func TestAPIAdminOrgCreate(t *testing.T) {
 		var apiOrg api.Organization
 		DecodeJSON(t, resp, &apiOrg)
 
-		assert.Equal(t, org.UserName, apiOrg.UserName)
+		assert.Equal(t, org.UserName, apiOrg.Name)
 		assert.Equal(t, org.FullName, apiOrg.FullName)
 		assert.Equal(t, org.Description, apiOrg.Description)
 		assert.Equal(t, org.Website, apiOrg.Website)