Bump pub to b21ac685bc64f6e81050ec0093aa83543d66e2fd

Changes:
```
> git log --format="%C(auto) %h %s" b297f1f..b21ac68
 https://dart.googlesource.com/pub.git/+/b21ac685b Bump actions/checkout from 5.0.0 to 6.0.0 in the github-actions group (4719)
 https://dart.googlesource.com/pub.git/+/65e289b95 Tag pattern: use hash of referenced commit not of tag itself (4707)

```

Diff: https://dart.googlesource.com/pub.git/+/b297f1f5e42f2569f15a98548f44b4b9a48487c1..b21ac685bc64f6e81050ec0093aa83543d66e2fd/
Change-Id: Iea51fe54d4bb7e3bb888483417b68f4175c1ddb8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/468280
Reviewed-by: Sarah Zakarias <[email protected]>
Commit-Queue: Sigurd Meldgaard <[email protected]>
Auto-Submit: Sigurd Meldgaard <[email protected]>
Commit-Queue: Sarah Zakarias <[email protected]>

Author: sigurdm

DEPS

@@ -142,7 +142,7 @@ vars = {
   "material_color_utilities_rev": "799b6ba2f3f1c28c67cc7e0b4f18e0c7d7f3c03e",
   "native_rev": "93039d7ef0b71cd01567e00e6b5be90a3e7d0668", # rolled manually while record_use is experimental
   "protobuf_rev": "9e30258e0aa6a6430ee36c84b75308a9702fde42",
-  "pub_rev": "b297f1f5e42f2569f15a98548f44b4b9a48487c1", # rolled manually
+  "pub_rev": "b21ac685bc64f6e81050ec0093aa83543d66e2fd", # rolled manually
   "shelf_rev": "dd830a0338b31bee92fe7ebc20b9bb963403b6b0",
   "sync_http_rev": "6666fff944221891182e1f80bf56569338164d72",
   "tar_rev": "13479f7c2a18f499e840ad470cfcca8c579f6909",