Update etherpad/etherpad Docker tag to v1.9.1

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
etherpad/etherpad	final	patch	1.9.0 -> 1.9.1

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Release Notes

ether/etherpad-lite

v1.9.1

Compare Source

Notable enhancements and fixes

• Security
  • Limit requested revisions in timeslider and export to head revision. (affects v1.9.0)
• Bugfixes
  • revisions in CHANGESET_REQ (timeslider) and export (txt, html, custom)
    are now checked to be numbers.
  • bump sql for audit fix
• Enhancements
  • Add keybinding meta-backspace to delete to beginning of line
  • Fix automatic Windows build via GitHub Actions
  • Enable docs to be build cross platform thanks to asciidoctor

Compatibility changes

• tests: drop windows 7 test coverage & use chrome latest for admin tests
• Require Node 16 for Etherpad and target Node 20 for testing

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