Remove support for Node.js 12 in CLI

Closes GH-1007.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

.github/workflows/main.yml

@@ -18,8 +18,8 @@ jobs:
     strategy:
       matrix:
         node:
-          - lts/erbium
-          - node
+          - lts/fermium
+          - lts/gallium
   canary:
     name: canary / ${{matrix.package}} / ${{matrix.node}} on ${{matrix.os}}
     runs-on: ${{matrix.os}}

packages/remark-cli/readme.md

@@ -47,7 +47,7 @@ If not, you can always use [`remark`][remark-core] itself manually in a script.
 ## Install
 
 This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c).
-In Node.js (version 12.20+, 14.14+, or 16.0+), install with [npm][]:
+In Node.js (version 14.14+, or 16.0+), install with [npm][]:
 
 ```sh
 npm install remark-cli
@@ -281,7 +281,7 @@ Earlier wins (so in the above file structure `folder/.remarkrc.js` wins over
 
 Projects maintained by the unified collective are compatible with all maintained
 versions of Node.js.
-As of now, that is Node.js 12.20+, 14.14+, and 16.0+.
+As of now, that is Node.js 14.14+, and 16.0+.
 Our projects sometimes work with older versions, but this is not guaranteed.
 
 ## Security