Elasticsearch 8.9.1 template

What about template update for Elasticsearch version 8.9.1 as sample version is throwing errors?

```
{
"template": "alertmanager-*",
"settings": {
"number_of_shards": 1,
"number_of_replicas": 1,
"index": {
"refresh_interval": "10s",
"query": {
"default_field": "groupLabels.alertname"
}
}
},
"mappings": {
"_default_": {
"_all": {
"enabled": false
},
"properties": {
"@timestamp": {
"type": "date",
"doc_values": true
}
},
"dynamic_templates": [
{
"string_fields": {
"match": "*",
"match_mapping_type": "string",
"mapping": {
"type": "string",
"index": "not_analyzed",
"ignore_above": 1024,
"doc_values": true
}
}
}
]
}
}
}
```

Locate the Elasticsearch index-template payload and the code path that submits it. Reproduce the template failure against Elasticsearch 8.9.1, then compare the template syntax and mappings with that version's requirements. Done means the sample template is accepted without errors and alertmanager data remains searchable.

Written by the indexing model from the issue text.