chore(release): remove package size reports

- Drop the afterAllArtifactBuild size reporting hook and the previous-release baseline downloads in CI
- Stop uploading, checksumming, and publishing package-size-*.json
- Keep the afterPack package content audit, now without size tracking
- Update RELEASING.md and the verification docs, and add a changelog fragment

package.json

@@ -269,8 +269,7 @@
         "from": "CHANGELOG.md",
         "to": "CHANGELOG.md"
       }
-    ],
-    "afterAllArtifactBuild": "scripts/package-audit.cjs"
+    ]
   },
   "patchedDependencies": {
     "@discordjs/rest@2.6.1": "patches/@discordjs%2Frest@2.6.1.patch"