Warn when mutating res if not streaming

[kara]

In #29010, we started throwing an error if the res was mutated after
getServerSideProps() returned. This was to support classic streaming,
where it would be possible to accidentally mutate the response headers
after they were already sent. However, this change also caught a few
non-streaming cases
that we don't want to break.

As such, with this change, we only throw the error if res is mutated
after gSSP returns and you've opted into using classic streaming.
Otherwise, we will only add a warning to the console.

[styfle]

Thanks!

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	kara/next.js mutate	Change
buildDuration	21.6s	19.4s	-2.2s
buildDurationCached	4s	4.4s	⚠️ +446ms
nodeModulesSize	475 MB	475 MB	⚠️ +561 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	kara/next.js mutate	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.725	3.506	-0.22
/ avg req/sec	671.23	713.12	+41.89
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.843	1.858	⚠️ +0.02
/error-in-render avg req/sec	1356.5	1345.89	⚠️ -10.61

Client Bundles (main, webpack, commons)

	vercel/next.js canary	kara/next.js mutate	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	27.7 kB	27.7 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	71.5 kB	71.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	kara/next.js mutate	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	kara/next.js mutate	Change
_app-HASH.js gzip	1.23 kB	1.23 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.44 kB	4.44 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	kara/next.js mutate	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	kara/next.js mutate	Change
index.html gzip	534 B	534 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	kara/next.js mutate	Change
buildDuration	17.4s	17.3s	-109ms
buildDurationCached	4s	4s	-11ms
nodeModulesSize	475 MB	475 MB	⚠️ +561 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	kara/next.js mutate	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.683	3.495	-0.19
/ avg req/sec	678.71	715.37	+36.66
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.871	1.849	-0.02
/error-in-render avg req/sec	1335.83	1351.88	+16.05

Client Bundles (main, webpack, commons)

	vercel/next.js canary	kara/next.js mutate	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	27.8 kB	27.8 kB	✓
webpack-HASH.js gzip	1.43 kB	1.43 kB	✓
Overall change	71.6 kB	71.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	kara/next.js mutate	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	kara/next.js mutate	Change
_app-HASH.js gzip	1.22 kB	1.22 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	621 B	621 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.9 kB	1.9 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	kara/next.js mutate	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	kara/next.js mutate	Change
index.html gzip	534 B	534 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 2730e4b