tidy-up: miscellaneous

tidy-up: miscellaneous

- sha256: fix backend priority in comment.
- URLs: link to IETF URLs to the HTML document, to match others.
- VERSIONS.md: use unified date format for recent entries too.
  Ref: https://github.com/curl/curl-www/commit/ce5d32032f8d3d8601f3ef022bbca485020d1bb9
- GHA/labeler.yml: alpha-sort file masks in a label block.
- tests/server/mqttd: fix call arg list in a disabled function.
- tests/server/mqttd: fix comment.

Closes #21473