use latest dart in github actions workflows

Author: robrbecker

.github/workflows/dart.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     container:
-      image: dart:2.17
+      image: dart
     steps:
     - uses: actions/checkout@v1
     - name: Install dependencies

.github/workflows/publish_demos.yml

@@ -7,7 +7,7 @@ jobs:
   build-and-deploy:
     runs-on: ubuntu-latest
     container:
-      image:  google/dart:latest
+      image: dart
     steps:
       - name: Checkout 🛎️
         uses: actions/[email protected]

.github/workflows/publish_release.yml

@@ -9,7 +9,7 @@ jobs:
 
     runs-on: ubuntu-latest
     container:
-      image: dart:2.17
+      image: dart
     steps:
       - name: Checkout
         uses: actions/checkout@v1

.github/workflows/release_unreleased_prs.yml

@@ -12,7 +12,7 @@ jobs:
   release:
     if: github.event.pull_request.merged == true || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
-    container: dart:2.17.7
+    container: dart
     permissions:
       contents: write