Update fixnum test SDK version from 3.1.0 to 3.4.4

Fixes package:test running dart2wasm.

Author: osa1

.github/workflows/fixnum.yaml

@@ -28,7 +28,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [3.1.0, dev]
+        sdk: [3.4.4, dev]
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8
       - uses: dart-lang/setup-dart@e51d8e571e22473a2ddebf0ef8a2123f0ab2c02c
@@ -55,7 +55,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [3.1.0, dev]
+        sdk: [3.4.4, dev]
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8
       - uses: dart-lang/setup-dart@e51d8e571e22473a2ddebf0ef8a2123f0ab2c02c