Bump version numbers and add upgrade guide for the 2.9.0 release

[nathandyer]

Status

Ready for review

Description of Changes

This PR bumps the version numbers throughout the documentation to 2.9.0, and adds an upgrade guide for this release.

I left the instructions for the Tails 6 upgrade in the guide since some folks may not have completed the upgrade as part of the previous release. If we feel these are not necessary I'm happy to remove them. Just let me know!

• Resolves Documentation update for 2.9.0 #571

Testing

• Visual review

Release

• We will need to tag a 2.9.0 stable docs release once 2.9.0 is out the door

Checklist (Optional)

• Doc linting (make docs-lint) passed locally
• Doc link linting (make docs-linkcheck) passed
• You have previewed (make docs) docs at http://localhost:8000

[eloquence]

LGTM, two nits inline. I think keeping the Tails 6 docs is a good idea for now; if we want to keep them a few months longer, it might be worth reorganizing them into their own section instead of copying them with each upgrade guide. But removing them with 2.10.0 is probably fine.

[nathandyer]

Thanks @eloquence, that sounds great. I think we're probably okay to remove them in 2.10.0, but we can decide one way or the other closer to that release once we have a better feel for how widely Tails 6 has been adopted.

[eloquence]

Good to go once release is live.

[nathandyer]

Thanks! After discussing with @zenmonkeykstop , I added an additional commit which mentions the new Tor PoW option, and links to our guide for enabling it.

Will need another review before it can be merged with the release.

[legoktm]

I only reviewed the most recent commit (the Tor PoW notice), and that LGTM. Approving based on Erik's prior approval.