Adding _meta.template_version to kibana reporting mappings (elastic#133846)

Part of elastic/kibana#231200 .

We realized that when the mappings are updated in a version upgrade,
the mappings are in fact updated, but not applied to the current
reporting datastream.

To fix this, we'll make a determination in Kibana that we should roll
over the data stream if the version in the template does not match the
version in the mappings of the datastream.  If they match, we don't need
to do anything.  If they don't match, or the version of the mappings is
not in the datastream (from before this PR), we will need to roll over the
data stream to apply the new mappings.

To make this happen, we need to add a new field to the mapping `_meta`,
which should match the template version.

references:

- https://www.elastic.co/docs/reference/elasticsearch/mapping-reference/mapping-meta-field

Co-authored-by: Elastic Machine <[email protected]>
(cherry picked from commit ebb94bd)

# Conflicts:
#	x-pack/plugin/stack/src/main/java/org/elasticsearch/xpack/stack/StackTemplateRegistry.java

Author: pmuellr

x-pack/plugin/core/template-resources/src/main/resources/[email protected]

@@ -10,6 +10,9 @@
   "template": {
     "lifecycle": {},
     "mappings": {
+      "_meta": {
+        "template_version": ${xpack.stack.template.version}
+      },
       "properties": {
         "meta": {
           "properties": {

x-pack/plugin/stack/src/main/java/org/elasticsearch/xpack/stack/StackTemplateRegistry.java

@@ -37,7 +37,7 @@ public class StackTemplateRegistry extends IndexTemplateRegistry {
 
     // The stack template registry version. This number must be incremented when we make changes
     // to built-in templates.
-    public static final int REGISTRY_VERSION = 16;
+    public static final int REGISTRY_VERSION = 18;
 
     public static final String TEMPLATE_VERSION_VARIABLE = "xpack.stack.template.version";
     public static final Setting<Boolean> STACK_TEMPLATES_ENABLED = Setting.boolSetting(