Revert standalone extension change for module type (#77978)

We don't need to change the extension as the main fix is to ensure the
`package.json` is copied to the standalone folder. The extension change
was for more correctness but can cause breakage for anyone using `type:
'module'` as they will need to update their flows for the new `.mjs`
extension. Since this is un-necessary breakage this reverts that piece
of the fix.

x-ref:
#77944 (comment)

Author: ijjk

packages/next/src/build/utils.ts

@@ -1687,7 +1687,7 @@ export async function copyTracedFiles(
   const serverOutputPath = path.join(
     outputPath,
     path.relative(tracingRoot, dir),
-    moduleType ? 'server.mjs' : 'server.js'
+    'server.js'
   )
   await fs.mkdir(path.dirname(serverOutputPath), { recursive: true })
 

test/production/standalone-mode/type-module/index.test.ts

@@ -41,7 +41,7 @@ describe('type-module', () => {
 
     expect(fs.existsSync(join(standalonePath, 'package.json'))).toBe(true)
 
-    const serverFile = join(standalonePath, 'server.mjs')
+    const serverFile = join(standalonePath, 'server.js')
     const appPort = await findPort()
     const server = await initNextServerScript(
       serverFile,