feat: rename etcd.ssl to etcd.ssl_options

[zhongwencool]

Fixes EMQX-9671
Keep all of SSL options consistent in the configuration file (not include the listener)

Summary

🤖 Generated by Copilot at 0b1a2dd

Refactored SSL configuration for etcd cluster and bumped emqx_conf version. Renamed ssl field to ssl_options in emqx_conf_schema.erl and added an alias for backward compatibility.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update