Use next.config.mjs for CNA templates

[ijjk]

As .mjs support for next.config is stable now we can swap our default to use it instead.

Closes NEXT-2052

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
buildDuration	12.9s	12.8s	N/A
buildDurationCached	7.2s	6.2s	N/A
nodeModulesSize	199 MB	199 MB	✓
nextStartRea..uration (ms)	434ms	433ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
193.HASH.js gzip	181 B	182 B	N/A
3f784ff6-HASH.js gzip	53.3 kB	53.3 kB	N/A
433-HASH.js gzip	28.5 kB	28.6 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	239 B	242 B	N/A
main-HASH.js gzip	31.8 kB	31.8 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	N/A
Overall change	45.2 kB	45.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
_app-HASH.js gzip	194 B	195 B	N/A
_error-HASH.js gzip	183 B	181 B	N/A
amp-HASH.js gzip	504 B	502 B	N/A
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.5 kB	2.5 kB	N/A
edge-ssr-HASH.js gzip	255 B	253 B	N/A
head-HASH.js gzip	350 B	349 B	N/A
hooks-HASH.js gzip	369 B	369 B	✓
image-HASH.js gzip	4.28 kB	4.28 kB	N/A
index-HASH.js gzip	255 B	256 B	N/A
link-HASH.js gzip	2.61 kB	2.61 kB	✓
routerDirect..HASH.js gzip	312 B	311 B	N/A
script-HASH.js gzip	385 B	383 B	N/A
withRouter-HASH.js gzip	307 B	308 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.4 kB	3.4 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
_buildManifest.js gzip	483 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
index.html gzip	528 B	527 B	N/A
link.html gzip	541 B	538 B	N/A
withRouter.html gzip	524 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
edge-ssr.js gzip	93.8 kB	93.8 kB	N/A
page.js gzip	148 kB	148 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
middleware-b..fest.js gzip	623 B	624 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	37.5 kB	37.5 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	2.07 kB	2.07 kB	✓

Next Runtimes

	vercel/next.js canary	ijjk/next.js update/cna-template-config	Change
app-page-exp...dev.js gzip	169 kB	169 kB	✓
app-page-exp..prod.js gzip	95.3 kB	95.3 kB	✓
app-page-tur..prod.js gzip	96 kB	96 kB	✓
app-page-tur..prod.js gzip	90.6 kB	90.6 kB	✓
app-page.run...dev.js gzip	142 kB	142 kB	✓
app-page.run..prod.js gzip	89.9 kB	89.9 kB	✓
app-route-ex...dev.js gzip	24.1 kB	24.1 kB	✓
app-route-ex..prod.js gzip	16.7 kB	16.7 kB	✓
app-route-tu..prod.js gzip	16.7 kB	16.7 kB	✓
app-route-tu..prod.js gzip	16.3 kB	16.3 kB	✓
app-route.ru...dev.js gzip	23.5 kB	23.5 kB	✓
app-route.ru..prod.js gzip	16.3 kB	16.3 kB	✓
pages-api-tu..prod.js gzip	9.38 kB	9.38 kB	✓
pages-api.ru...dev.js gzip	9.65 kB	9.65 kB	✓
pages-api.ru..prod.js gzip	9.37 kB	9.37 kB	✓
pages-turbo...prod.js gzip	21.9 kB	21.9 kB	✓
pages.runtim...dev.js gzip	22.5 kB	22.5 kB	✓
pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	49.5 kB	49.5 kB	✓
Overall change	940 kB	940 kB	✓

Commit: 46d801e

[samcx]