services: remove assertion on 'task' field being set (#14864)

This PR removes the assertion around when the 'task' field of
a check may be set. Starting in Nomad 1.4 we automatically set
the task field on all checks in support of the NSD checks feature.

This is causing validation problems elsewhere, e.g. when a group
service using the Consul provider sets 'task' it will fail
validation that worked previously.

The assertion of leaving 'task' unset was only about making sure
job submitters weren't expecting some behavior, but in practice
is causing bugs now that we need the task field for more than it
was originally added for.

We can simply update the docs, noting when the task field set by
job submitters actually has value.

.changelog/14864.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+services: Fixed a regression where check task validation stopped allowing some configurations
+```

nomad/structs/structs.go

@@ -6681,10 +6681,6 @@ func (tg *TaskGroup) validateServices() error {
 
 		for _, check := range service.Checks {
 			if check.TaskName != "" {
-				if check.Type != ServiceCheckScript && check.Type != ServiceCheckGRPC {
-					mErr.Errors = append(mErr.Errors,
-						fmt.Errorf("Check %s invalid: only script and gRPC checks should have tasks", check.Name))
-				}
 				if check.AddressMode == AddressModeDriver {
 					mErr.Errors = append(mErr.Errors, fmt.Errorf("Check %q invalid: cannot use address_mode=\"driver\", only checks defined in a \"task\" service block can use this mode", service.Name))
 				}

nomad/structs/structs_test.go

@@ -1278,9 +1278,6 @@ func TestTaskGroup_Validate(t *testing.T) {
 	expected = `Check check-a invalid: refers to non-existent task task-b`
 	require.Contains(t, err.Error(), expected)
 
-	expected = `Check check-a invalid: only script and gRPC checks should have tasks`
-	require.Contains(t, err.Error(), expected)
-
 	tg = &TaskGroup{
 		Name: "group-a",
 		Services: []*Service{

website/content/docs/job-specification/check.mdx

@@ -134,11 +134,11 @@ job "example" {
 - `protocol` `(string: "http")` - Specifies the protocol for the http-based
   health checks. Valid options are `http` and `https`.
 
-- `task` `(string: <required>)` - Specifies the task associated with this
+- `task` `(string: "")` - Specifies the task associated with this
   check. Scripts are executed within the task's environment, and
-  `check_restart` stanzas will apply to the specified task. For `checks` on group
+  `check_restart` stanzas will apply to the specified task. Inherits
-  level `services` only. Inherits the [`service.task`][service_task] value if not
+  the [`service.task`][service_task] value if not set. Must be unset
-  set. May only be set for script or gRPC checks.
+  or equivelent to `service.task` in task-level services.
 
 - `timeout` `(string: <required>)` - Specifies how long to wait for a health check
   query to succeed. This is specified using a label suffix like "30s" or "1h". This