ci!: Drop support for Node.js 16. Add initial support for Node.js 20 (#6649)

We need to drop node 16 support, [as support for it is ends much earlier now, due to support for openssl 1.1.1 ending](https://nodejs.org/en/blog/announcements/nodejs16-eol).

`0.236.x` releases will continue to support Node.js 16 for another two months, and `1.x.x` releases will only support Node.js 18 for now.

.github/workflows/docker-base-image.yml

@@ -7,10 +7,11 @@ on:
         description: 'Node.js version to build this image with.'
         type: choice
         required: true
-        default: '16'
+        default: '18'
         options:
           - '16'
           - '18'
+          - '20'
 
 jobs:
   build: