Move Node.js 8 alongside other legacy Node.js versions in CI

Now that Node.js 8 is end-of-life / no longer maintained, we might as
well treat it as a legacy Node.js version too.

.github/workflows/verify.yml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [8.x, 10.x, 12.x, 14.x, 15.x]
+        node-version: [10.x, 12.x, 14.x, 15.x]
 
     steps:
       - uses: actions/checkout@v2
@@ -28,7 +28,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [0.10.x, 0.12.x, 4.x, 6.x]
+        node-version: [0.10.x, 0.12.x, 4.x, 6.x, 8.x]
 
     steps:
       - uses: actions/checkout@v2