build: add v14 to the test matrix

Author: mmarchini

.github/workflows/push.yml

@@ -19,8 +19,9 @@ jobs:
           - version: 10.x
           - version: 12.x
           - version: 14.x
+          - version: 15.x
             mirror: https://nodejs.org/download/nightly
-          - version: 14.x
+          - version: 15.x
             mirror: https://nodejs.org/download/v8-canary
         os: [ubuntu-latest, macos-latest]
     steps: