Upgrade to Debian Bullseye and Node 14

Summary: Drop Python dependency as not needed anymore.

Test Plan:

• Build image with docker build -t nasqueron/etherpad .
• Deploy image on docker-001 for pad.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2590