release-process documentation is out of date

[cfm]

The "Release Process" section is out of date in a few ways, based on my experience in freedomofpress/securedrop#7790:

• Building securedrop-admin via make build-debs-admin is not documented at all.
• There are still references to Ubuntu Focal, whereas make build-debs now targets Noble and make build-debs-admin targets Tails's underlying Debian trixie.
• While it is true that "-dbgsym packages belong in the main-debug component repo", this is handled automatically by tools/publish; it's not something that you have to make happen when you commit the packages themselves.
• The "Build Metadata" page could be updated now that logs are saved automatically and can be committed directly.