Change Node.js requirement in README (mastodon#16246)

Author: Gargron

README.md

@@ -70,7 +70,7 @@ Mastodon acts as an OAuth2 provider so 3rd party apps can use the REST and Strea
 - **PostgreSQL** 9.5+
 - **Redis** 4+
 - **Ruby** 2.5+
-- **Node.js** 10.13+
+- **Node.js** 12+
 
 The repository includes deployment configurations for **Docker and docker-compose**, but also a few specific platforms like **Heroku**, **Scalingo**, and **Nanobox**. The [**stand-alone** installation guide](https://docs.joinmastodon.org/admin/install/) is available in the documentation.