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: ce5d32032f
- 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

.github/labeler.yml

@@ -548,9 +548,9 @@ Windows:
               lib/curlx/fopen.*,\
               lib/curlx/multibyte.*,\
               lib/curlx/winapi.*,\
+              lib/libcurl.def,\
               lib/vtls/schannel*,\
               m4/curl-schannel.m4,\
               projects/Windows/**,\
-              src/tool_doswin.c,\
-              lib/libcurl.def\
+              src/tool_doswin.c\
               }"