Remove legacy xpack references

[joshuarrrr]

Our config file still references xpack settings:

OpenSearch-Dashboards/config/opensearch_dashboards.yml

Lines 61 to 65 in 7b18e06

	# Optional settings that provide the paths to the PEM-format SSL certificate and key files.
	# These files are used to verify the identity of OpenSearch Dashboards to OpenSearch and are required when
	# xpack.security.http.ssl.client_authentication in OpenSearch is set to required.
	#opensearch.ssl.certificate: /path/to/your/client.crt
	#opensearch.ssl.key: /path/to/your/client.key

There are other references still scattered around: https://github.com/search?q=repo%3Aopensearch-project%2FOpenSearch-Dashboards+xpack&type=code&p=1

[Aigerim-ai]

Can you please assign this issue to me?

[joshuarrrr]

@Aigerim-ai I assigned to you, but we may need to get some additional context first. Config file changes also need to be done in a backwards compatible way, so that old values are correctly re-written to new values.