docs(prerequisites): update to Node.js v16

Updates the Node.js version installed by the prerequisite instructions
from v14 to v16, which is the latest LTS release.

The instructions for installing the Node Version Manager (NVM) have
also been updated for v0.39.0 (previously v0.38.0).

Change-Id: I85528b3906305914ba6169b4dc5aafcf5b36a339
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed