ci: unpin setup-node to v2

Author: nschonni

.github/workflows/doctoc.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-node@v2.5.1
+      - uses: actions/setup-node@v2
         with:
           node-version: '12.x'
       - name: Install doctoc

.github/workflows/eclint.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-node@v2.5.1
+      - uses: actions/setup-node@v2
         with:
           node-version: '12.x'
       - run: npm i -g eclint

.github/workflows/markdown-link-check.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-node@v2.5.1
+      - uses: actions/setup-node@v2
         with:
           node-version: '12.x'
       - name: Install markdown-link-check