Update workflows to use latest release instead of master branch

.github/workflows/autoprefixer.yml

@@ -8,12 +8,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout to latest commit
-        uses: actions/checkout@v2
+        uses: actions/checkout@v2.3.3
 
       - name: Setup Node
         uses: actions/setup-node@v2.1.2
         with:
-          node-version: "12.x"
+          node-version: "15.x"
 
       - name: Run autoprefixer
         run: |
@@ -21,7 +21,7 @@ jobs:
           npm run autoprefixer
 
       - name: Create Pull Request
-        uses: peter-evans/create-pull-request@v3
+        uses: peter-evans/create-pull-request@v3.4.1
         with:
           branch: autoprefixer
           branch-suffix: timestamp