Move to newer base image for end-to-end tests
Move to newer base image for end-to-end tests

This updates to Node 14 (current LTS) as well as moving from Debian Stretch to
Buster for the base OS. The Debian upgrade brings along a newer Python 3.8,
which is actively supported.

Fixes https://github.com/vector-im/element-web/issues/16272

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