Migrate docs repositories to bookworm

[legoktm]

Describe the change

Our docs repositories are currently built on Debian bullseye / Python 3.9, we should migrate them to bookworm as we're doing the same with the rest of our stack, and because we're starting to hit issues with 3.9 being less well supported upstream (e.g. https://github.com/sphinx-doc/sphinx-autobuild/issues/155).

• [x] securedrop-dev-docs: https://github.com/freedomofpress/securedrop-dev-docs/pull/164
• [x] securedrop-workstation-docs: https://github.com/freedomofpress/securedrop-workstation-docs/pull/220
• [x] securedrop-docs: https://github.com/freedomofpress/securedrop-docs/pull/574

[eloquence]

Unless you or someone else want to, I can try to get this over the finish line this week, @legoktm - looks like you've done most of the legwork already though, so understand if you prefer to see it through yourself!

[legoktm]

Go for it, I can do reviews then :)

[legoktm]

All done, thanks @eloquence!