refactor: use POST not PUT for /users/{name}/change_pwd

[sstrigler]

Fixes EMQX-8373

If your build fails due to your commit message not passing the build checks, please review the guidelines here: https://github.com/emqx/emqx/blob/master/CONTRIBUTING.md.

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/ dir
• For internal contributor: there is a jira ticket to track this change
• sync with dashboard team

[coveralls]

Pull Request Test Coverage Report for Build 3684366512

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 26 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.01%) to 79.347%

---

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_gateway/src/mqttsn/emqx_sn_frame.erl	1	64.04%
apps/emqx/src/emqx_banned.erl	1	89.39%
apps/emqx/src/emqx_config.erl	1	84.36%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_message.erl	1	94.83%
apps/emqx/src/emqx_session.erl	1	87.3%
apps/emqx/src/emqx_config_handler.erl	2	96.21%
apps/emqx/src/emqx_channel.erl	3	86.92%
apps/emqx/src/emqx_broker.erl	5	82.42%
apps/emqx/src/emqx_reason_codes.erl	5	94.12%

Totals
Change from base Build 3676494261:	0.01%
Covered Lines:	22160
Relevant Lines:	27928

---

💛 - Coveralls