Bump pub to 21aa23f148a428ae4dd97a27ecc70b08c1328089

Changes:
```
> git log --format="%C(auto) %h %s" 4fd2051..21aa23f
 https://dart.googlesource.com/pub.git/+/21aa23f1 Revert "Create output folder if it doesn't exist in the precompiler (#3676)" (#3677)
 https://dart.googlesource.com/pub.git/+/04f7bce8 Don't consider resolution outdated in dev-releases (#3678)
 https://dart.googlesource.com/pub.git/+/74473e12 Create output folder if it doesn't exist in the precompiler (#3676)
 https://dart.googlesource.com/pub.git/+/5bfd68b1 Avoid .incremental.dill.incremental.dill in temporary file name (#3675)
 https://dart.googlesource.com/pub.git/+/c3870bc1 Unskip test (#3671)

```

Diff: https://dart.googlesource.com/pub.git/+/4fd2051bb8060ea2f2e02b13dfeb2f793d197994..21aa23f148a428ae4dd97a27ecc70b08c1328089/
Change-Id: Ie5060563cc3f96b0bc52e542f7a7b19d17fefd8f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/273223
Reviewed-by: Jonas Jensen <jonasfj@google.com>
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>

DEPS

@@ -152,7 +152,7 @@ vars = {
   "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
   "pool_rev": "1ea5b031cfda37786d305292cb8104dffb45d9ae",
   "protobuf_rev": "c181573c330ff0337db0bab5501deda3a42ba028",
-  "pub_rev": "4fd2051bb8060ea2f2e02b13dfeb2f793d197994", # manually rev'd
+  "pub_rev": "21aa23f148a428ae4dd97a27ecc70b08c1328089", # manually rev'd
   "pub_semver_rev": "17231113f98c80285881cc4452863bcfd085d1a8",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "shelf_rev": "1c2104737973715426035c11ba840c7f23d8f186",