.drone.yml: Run on current and LTS Node.js releases

Node.js v10 will cease to be LTS next month and I haven't even used
it for a while already, so let's change this to only run on 12, 14,
and 15, which are the current active and LTS releases.

See: https://nodejs.org/en/about/releases/

.drone.yml

@@ -1,17 +1,5 @@
 kind: pipeline
 type: docker
-name: node10
-
-steps:
-- name: build
-  image: node:10-alpine
-  commands:
-  - npm install
-  - npm run build
-
----
-kind: pipeline
-type: docker
 name: node12
 
 steps:
@@ -32,3 +20,15 @@ steps:
   commands:
   - npm install
   - npm run build
+
+---
+kind: pipeline
+type: docker
+name: node15
+
+steps:
+- name: build
+  image: node:15-alpine
+  commands:
+  - npm install
+  - npm run build