chore: start deprecating mcast cluster discovery

[zmstone]

Fixes EMQX-9563

Summary

🤖 Generated by Copilot at bdcc069

The pull request removes the mcast cluster discovery strategy from the configuration schema and the user interface, and warns the users about its deprecation. This change is part of the effort to simplify and improve the cluster management features.

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
  Delete mcast cluster discovery from docs emqx-docs#1920
• 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 5177464363

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 36 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.03%) to 81.638%

---

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_ft/src/emqx_ft_storage_exporter_fs.erl	1	82.22%
apps/emqx_gateway/src/emqx_gateway_schema.erl	1	88.89%
apps/emqx_management/src/emqx_mgmt_util.erl	1	13.79%
apps/emqx/src/emqx_broker.erl	1	88.2%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_quic_data_stream.erl	1	75.21%
apps/emqx/src/emqx_ws_connection.erl	1	77.2%
apps/emqx_dashboard/src/emqx_dashboard_monitor.erl	2	63.89%
apps/emqx_ft/src/emqx_ft_storage_fs.erl	2	81.29%
apps/emqx/src/emqx_channel.erl	2	87.69%

Totals
Change from base Build 5175822785:	-0.03%
Covered Lines:	28939
Relevant Lines:	35448

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💛 - Coveralls

[yanzhiemq]

Bug Fixes

• Deprecated UDP mcast mechanism for cluster discovery.
  This feature has been planed for deprecation since 5.0 mainly due to the lack of actual production use. This feature code is not yet removed in 5.1, but the document interface is demoted.

[yanzhiemq]

修复

• 弃用了集群发现的 UDP 组播机制。
  该功能自5.0版本以来一直计划弃用，主要是因为实际生产中缺乏使用。尽管该功能的代码在5.1版本中尚未移除，但文档接口已经被降级处理。