Update Etherpad version to latest

[ilmartyrk]

We need to update Etherpad to use the latest version.

We also need to update the authentication plugins and check that all the features including html export work as expected

[BeccaMelhuish]

@ilmartyrk I'm thinking we should sort this soon due to this issue which is recurring in live and quite serious: citizenos/citizenos-fe#2199

Am I right in understanding this is a big bit of work? How many days would you estimate?

[BeccaMelhuish]

Ilmar has it locally but needs to be sure all is working OK to make it live. Risky.