docs(readme): update elastic version

README.md

@@ -41,7 +41,7 @@ If this is the first time the stack is started, you'll have to create a Logstash
 ```posh
 PS> $Headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
 PS> $Headers.Add("Content-Type", "application/json")
-PS> $Headers.Add("kbn-version", "7.17.0")
+PS> $Headers.Add("kbn-version", "8.18.8")
 PS> $Auth = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes("elastic:changeme"))
 PS> $Headers.Add("Authorization", "Basic {0}" -f $Auth)
 PS> Invoke-RestMethod "http://localhost:5601/api/saved_objects/index-pattern" `
@@ -81,7 +81,7 @@ If this is the first time the stack is started, you'll have to create a Logstash
 ```bash
 $ curl -XPOST -D- 'http://localhost:5601/api/saved_objects/index-pattern' \
     -H 'Content-Type: application/json' \
-    -H 'kbn-version: 7.17.0' \
+    -H 'kbn-version: 8.18.8' \
     -u elastic:changeme \
     -d '{"attributes":{"title":"logstash-*","timeFieldName":"@timestamp"}}'
 ```