chore: deprecate statsd

[HJianBo]

Fixes https://emqx.atlassian.net/browse/EMQX-9313

We have decided to abolish this feature, so in version 5.0 we have hidden the configuration document and marked the related HTTP API as deprecated.

Summary

🤖 Generated by Copilot at 712f7b7

Deprecated the statsd plugin and hid its configuration from the hocon schema. Modified emqx_statsd_api.erl and emqx_statsd_schema.erl to reflect this change.

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 4751198620

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 44 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.04%) to 81.239%

---

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_bridge/src/emqx_bridge.erl	1	91.27%
apps/emqx_gateway_mqttsn/src/emqx_mqttsn_channel.erl	1	73.13%
apps/emqx/src/emqx_broker.erl	1	85.96%
apps/emqx/src/emqx_flapping.erl	1	87.27%
apps/emqx/src/emqx_logger_textfmt.erl	1	59.09%
apps/emqx_resource/src/emqx_resource_buffer_worker.erl	2	94.57%
apps/emqx/src/emqx_cm.erl	2	91.44%
apps/emqx/src/emqx_session.erl	2	87.25%
apps/emqx/src/emqx_stats.erl	2	91.67%
apps/emqx_gateway_exproto/src/emqx_exproto_gcli.erl	3	48.78%

Totals
Change from base Build 4745034971:	-0.04%
Covered Lines:	25596
Relevant Lines:	31507

---

💛 - Coveralls