[keyserver] Update node version to 16.18.0

Summary:
This is the final step for ENG-929
(https://linear.app/comm/issue/ENG-929/fix-issues-with-node-1614-import-syntax-assertions).
I changed the node version in keyserver/.nvmrc and the tag in the first
line of keyserver/Dockerfile.

Test Plan:
After changing the version, I ran yarn cleaninstall, and
tested yarn dev and yarn prod in both web and landing, yarn dev
in native, and yarn prod-build followed by yarn prod in
keyserver. All of the above run successfully.

Reviewers: ashoat, atul, abosh

Reviewed By: ashoat

Subscribers: tomek, ginsu

Differential Revision: https://phab.comm.dev/D5467