refactor: delete UDP multicast clustering strategy

[zmstone]

Fixes EMQX-11546

Release version: v/e5.4.0

Summary

mcast clustering strategy was announced deprecated since 5.1, now it's the time to finally drop the support from codebase.

Schema change is NOT backward compatible.

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
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket docs: delete UDP mcast clustering emqx-docs#2253
• 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