Bump SDK constraints for pub (dart-lang/pool#45)

Use a 2.12.0 lower bound since pub does not understand allowed
experiments for earlier versions.

Use a 3.0.0 upper bound to avoid a warning in pub and to give some
flexibility in publishing for stable.
diff --git a/pkgs/pool/CHANGELOG.md b/pkgs/pool/CHANGELOG.md
index 035605e..6fd755b 100644
--- a/pkgs/pool/CHANGELOG.md
+++ b/pkgs/pool/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 1.5.0-nullsafety.3
+
+* Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release
+  guidelines.
+
 ## 1.5.0-nullsafety.2
 
 * Allow prerelease versions of the 2.12 sdk.
diff --git a/pkgs/pool/pubspec.yaml b/pkgs/pool/pubspec.yaml
index 5e0a69b..67d8b6c 100644
--- a/pkgs/pool/pubspec.yaml
+++ b/pkgs/pool/pubspec.yaml
@@ -1,5 +1,5 @@
 name: pool
-version: 1.5.0-nullsafety.2
+version: 1.5.0-nullsafety.3
 
 description: >-
   Manage a finite pool of resources.
@@ -7,8 +7,7 @@
 homepage: https://github.com/dart-lang/pool
 
 environment:
-  # This must remain a tight constraint until nnbd is stable
-  sdk: '>=2.10.0-0 <2.12.0'
+  sdk: ">=2.12.0-0 <3.0.0"
 
 dependencies:
   async: '>=2.5.0-nullsafety <2.5.0'