Make polyfill version validation a warning

[mischnic]

Change code from #8387 to a warning instead, as I'm not really confident that there are no false-negatives (cases where the user wants to override themselves)

$ parcel build index.js

@parcel/resolver-default: Could not find module "buffer" satisfying ^5.5.0.

  /.../parcel/tests/builtins-monorepo/package.json:7:3
    6 |   "devDependencies": {
  > 7 |     "buffer": "^4.0.0"
  >   |     ^^^^^^^^ Found this conflicting local requirement.
    8 |   }
    9 | }

✨ Built in 1.50s

dist/index.js    63.74 KB    42ms

[parcel-benchmark]

Benchmark Results

Kitchen Sink ✅

Timings

Description	Time	Difference
Cold	1.43s	-7.00ms
Cached	374.00ms	+13.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

Bundle	Size	Difference	Time	Difference
dist/legacy/index.2c76ad23.js	1.66kb	+0.00b	413.00ms	-26.00ms 🚀
dist/legacy/index.8aaa89c9.js	1.20kb	+0.00b	413.00ms	-27.00ms 🚀
dist/modern/index.6be20f01.js	1.13kb	+0.00b	413.00ms	-27.00ms 🚀

React HackerNews ✅

Timings

Description	Time	Difference
Cold	9.46s	+9.46s ⚠️
Cached	471.00ms	+472.00ms ⚠️

Cold Bundles

No bundles found, this is probably a failed build...

Cached Bundles

No bundles found, this is probably a failed build...

AtlasKit Editor ✅

Timings

Description	Time	Difference
Cold	1.70m	+1.70m ⚠️
Cached	2.79s	+2.79s ⚠️

Cold Bundles

No bundles found, this is probably a failed build...

Cached Bundles

No bundles found, this is probably a failed build...

Three.js ✅

Timings

Description	Time	Difference
Cold	6.78s	-31.00ms
Cached	293.00ms	-30.00ms 🚀

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

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