fix(config): add back reconnect_interval field as deprecated

[zmstone]

Fixes https://emqx.atlassian.net/browse/EMQX-8920

It was removed by mistake in this commit: c5a7cd5
as a part of
#9870

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/<version>/(feat|fix)-<PR-id>.en.md and .zh.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