Fix sitemap generateSitemaps support for string id

[huozhi]

What

Fixes the string id that broken when sitemap is optimized to static route.

Why

When sitemap is optimized to static route in production, the route argument is changed from [[...__metadata_id__]] to [__metadata_id__], so the type of it is also changed from array to string that should reflect in the loader code.

Fixes #60894
Closes NEXT-2154

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	Change
buildDuration	11.7s	11.7s	N/A
buildDurationCached	6s	5s	N/A
nodeModulesSize	200 MB	200 MB	⚠️ +800 B
nextStartRea..uration (ms)	424ms	431ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	Change
193.HASH.js gzip	181 B	182 B	N/A
3f784ff6-HASH.js gzip	53.4 kB	53.4 kB	✓
411-HASH.js gzip	29.8 kB	29.8 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	240 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	98.6 kB	98.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	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	4.18 kB	4.18 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	vercel/next.js fix/sitemap-ids	Change
_buildManifest.js gzip	484 B	485 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	Change
index.html gzip	527 B	526 B	N/A
link.html gzip	539 B	538 B	N/A
withRouter.html gzip	522 B	521 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	Change
edge-ssr.js gzip	94 kB	94 kB	N/A
page.js gzip	149 kB	149 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js fix/sitemap-ids	Change
middleware-b..fest.js gzip	624 B	627 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	37.6 kB	37.6 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	vercel/next.js fix/sitemap-ids	Change
app-page-exp...dev.js gzip	170 kB	170 kB	✓
app-page-exp..prod.js gzip	95.6 kB	95.6 kB	✓
app-page-tur..prod.js gzip	96.3 kB	96.3 kB	✓
app-page-tur..prod.js gzip	90.9 kB	90.9 kB	✓
app-page.run...dev.js gzip	142 kB	142 kB	✓
app-page.run..prod.js gzip	90.2 kB	90.2 kB	✓
app-route-ex...dev.js gzip	22.2 kB	22.2 kB	✓
app-route-ex..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.5 kB	14.5 kB	✓
app-route.ru...dev.js gzip	21.6 kB	21.6 kB	✓
app-route.ru..prod.js gzip	14.5 kB	14.5 kB	✓
pages-api-tu..prod.js gzip	9.41 kB	9.41 kB	✓
pages-api.ru...dev.js gzip	9.68 kB	9.68 kB	✓
pages-api.ru..prod.js gzip	9.4 kB	9.4 kB	✓
pages-turbo...prod.js gzip	22 kB	22 kB	✓
pages.runtim...dev.js gzip	22.6 kB	22.6 kB	✓
pages.runtim..prod.js gzip	22 kB	22 kB	✓
server.runti..prod.js gzip	49.7 kB	49.7 kB	✓
Overall change	932 kB	932 kB	✓

Commit: a630b3a