Rename safeOutputFile to atomicOverwriteFile (#1167)

"safe" suggested the data being written survives a crash, it only
guarantees the old file is either kept or fully replaced. The new name
says what it does. No alias, plugin API breaks are allowed in minor
versions and the 2.1 migration guide lists the rename.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Cooldude2606 <25043174+Cooldude2606@users.noreply.github.com>

docs/devs/plugin-migration-to-2.1.md

@@ -61,6 +61,12 @@ The `container` property of `WebPluginContext` is now the plugin's ES module nam
 `lib.PluginWebpackEnvInfo` is renamed to `lib.PluginWebEnvInfo`.
 
 
+## Renamed lib functions
+
+`lib.safeOutputFile` is renamed to `lib.atomicOverwriteFile`.
+It takes the same arguments.
+
+
 ## Host pairing
 
 Host tokens are no longer generated by users.