fix: copy cluster-override.conf from old version

[zhongwencool]

Fixes #10474
EMQX-9680

Previously, when we copied the initial configuration from an old node, if there was no 'has_deprecated_file' field, we treated it as a new mechanism using cluster.hocon. However, the old node never returned the has_deprecated_file' field before upgrading (due to a problem with the old code), so during rolling upgrade, it was always assumed that cluster.hocon was in effect when it should have been cluster-override.conf.

It's hard to write rolling upgrade CT test. I have verify on v5.0.21 and v5.0.23

Summary

🤖 Generated by Copilot at 7af9c18

Improved cluster configuration synchronization for mixed-version nodes. Refactored emqx_conf_app.erl and bumped emqx_conf version to 0.1.18.

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

[coveralls]

Pull Request Test Coverage Report for Build 4783657330

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• 40 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.005%) to 81.188%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx_conf/src/emqx_conf_app.erl	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx/src/emqx_broker.erl	1	88.2%
apps/emqx/src/emqx_channel.erl	1	87.64%
apps/emqx/src/emqx_connection.erl	1	85.15%
apps/emqx/src/emqx_listeners.erl	1	86.81%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_message.erl	1	94.83%
apps/emqx/src/emqx_quic_data_stream.erl	1	75.21%
apps/emqx_resource/src/emqx_resource_manager.erl	2	88.56%
apps/emqx/src/emqx_cm.erl	2	90.99%
apps/emqx_resource/src/emqx_resource_buffer_worker.erl	3	94.42%

Totals
Change from base Build 4777157010:	-0.005%
Covered Lines:	25691
Relevant Lines:	31644

---

💛 - Coveralls

[zmstone]

Writeoff for merge without test case.
Please make sure that:

1. The change has been tested (manually?)
2. There is a follow up task to add test case.

[zhongwencool]

Writeoff for merge without test case. Please make sure that:

1. The change has been tested (manually?)
2. There is a follow up task to add test case.

follow up task: https://emqx.atlassian.net/browse/EMQX-9690