chore: update etherpad to v2.5.1

In addition to the etherpad update, this replaces ep_auth_session with our own fork. etherpad 2.5.0 breaks compatibility with ep_auth_session and ep_auth_session is unmaintained, see !13 (comment 254776).

Closes #22
Closes #21
Closes #20
Closes #19
Closes #18
Closes #17
Closes #16
Requires https://git.opentalk.dev/opentalk/backend/libs/ep_ot_auth_session/-/merge_requests/1
Supersedes !13 (closed)