[BUG] Update compatibility.md

[shoustech]

Describe the bug
Currently the compatibility file in this repo lists Logstash 7.13.2 as the only compatible version of Logstash that this plugin can be ran with. Opensearch documentation does not list a compatible version but does use Logstash 8.8 in the example. In the repositories change logs it is mentioned that logstash-oss was upgraded to 8.12.1. As a user this is confusing on what version of Logstash I should be running and I believe this can be fixed by updating the compatibility documentation file.

Expected behavior
By looking at the compatibility.md file I should be able to identify what the newest version of logstash-oss is supported with the logstash-output-opensearch plugin.

Additional context
Relevant open issue #141

[dblock]

Thanks @shoustech, please do contribute an update!