Switch to .cjs PostCSS config (#58380)

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### What?

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Closes NEXT-
Fixes #

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### What?

Prevent confusing error messages when changing to `"type": "module"` in
`package.json`

```
./node_modules/next/dist/build/webpack/loaders/css-loader/src/index.js??ruleSet[1].rules[2].oneOf[8].use[1]!./node_modules/next/dist/build/webpack/loaders/postcss-loader/src/index.js??ruleSet[1].rules[2].oneOf[8].use[2]!./src/styles/index.css
Error [ERR_REQUIRE_ESM]: require() of ES Module /path/to/my/repo/components/postcss.config.js from /path/to/my/repo/components/node_modules/next/dist/lib/find-config.js not supported.
Instead change the require of postcss.config.js in /path/to/my/repo/components/node_modules/next/dist/lib/find-config.js to a dynamic import() which is available in all CommonJS modules.
```

### Why?

Even though PostCSS itself [supports ESM and TypeScript configuration
files](postcss/postcss-load-config#230),
Next.js itself does not (because of `next/lib/find-config`):

- #34448

### How?

By switching to `.cjs`, the config will stay recognized as CommonJS even
after switching to `"type": "module"` in `package.json`

cc @balazsorban44

---------

Co-authored-by: Sam Ko <[email protected]>

packages/create-next-app/templates/index.ts

@@ -51,7 +51,7 @@ export const installTemplate = async ({
   if (!tailwind)
     copySource.push(
       mode == "ts" ? "tailwind.config.ts" : "!tailwind.config.js",
-      "!postcss.config.js",
+      "!postcss.config.cjs",
     );
 
   await copy(copySource, root, {

test/integration/create-next-app/lib/specification.ts

@@ -68,7 +68,7 @@ export const projectSpecification: ProjectSpecification = {
         'pages/_app.js',
         'pages/api/hello.js',
         'pages/index.js',
-        'postcss.config.js',
+        'postcss.config.cjs',
         'tailwind.config.js',
       ],
       deps: [],
@@ -80,7 +80,7 @@ export const projectSpecification: ProjectSpecification = {
         'pages/_app.tsx',
         'pages/api/hello.ts',
         'pages/index.tsx',
-        'postcss.config.js',
+        'postcss.config.cjs',
         'tailwind.config.ts',
         'tsconfig.json',
       ],
@@ -125,7 +125,7 @@ export const projectSpecification: ProjectSpecification = {
         'app/layout.js',
         'app/page.js',
         'jsconfig.json',
-        'postcss.config.js',
+        'postcss.config.cjs',
         'tailwind.config.js',
       ],
     },
@@ -143,7 +143,7 @@ export const projectSpecification: ProjectSpecification = {
         'app/layout.tsx',
         'app/page.tsx',
         'next-env.d.ts',
-        'postcss.config.js',
+        'postcss.config.cjs',
         'tailwind.config.ts',
         'tsconfig.json',
       ],

test/integration/create-next-app/lib/utils.ts

@@ -135,7 +135,7 @@ export const shouldBeTemplateProject = ({
     files: getProjectSetting({ template, mode, setting: 'files', srcDir }),
   })
 
-  // Tailwind templates share the same files (tailwind.config.js, postcss.config.js)
+  // Tailwind templates share the same files (tailwind.config.js, postcss.config.cjs)
   if (template !== 'app-tw' && template !== 'default-tw') {
     projectFilesShouldNotExist({
       cwd,