[Fleet / self managed] Error dialing x509: certificate signed by unknown authority | Fleet server not able to connect with elasticsearch

Elastic - Kibana Version: 8.4.2
OS - Ubuntu 22
Installation - Self managed

**Steps to Reproduce:**
- install Elastic and Kibana
- reversed proxy used to access Kibana
- login via Elastic user
- go to management => fleet => add fleet server => advanced => follow steps
- go to fleet server => install elastic agent with certificates
- installation is successful
- go to either metricbeat or filebeat logs
- search the logs for connection failed or established
- error: `Error dialing x509: certificate signed by unknown authority`

**Expected result**
- fleet should establish the connection with Elasticsearch

for details please check:
https://discuss.elastic.co/t/elastic-agent-error-dialing-x509-certificate-signed-by-unknown-authority/315675