CI: Remove Node 15 from matrix

Node 15 is EOL since 01 June 2021

See https://endoflife.date/nodejs

Signed-off-by: David Mehren <git@herrmehren.de>

.github/workflows/node.js.yml

@@ -39,7 +39,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [12.x, 14.x, 15.x, 16.x]
+        node-version: [12.x, 14.x, 16.x]
     steps:
       - uses: actions/checkout@v2
       - name: Use Node.js ${{ matrix.node-version }}
@@ -63,7 +63,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [12.x, 14.x, 15.x, 16.x]
+        node-version: [12.x, 14.x, 16.x]
     steps:
       - uses: actions/checkout@v2
       - name: Use Node.js ${{ matrix.node-version }}