backports: revisit decision on whether to enable by default for bullseye

As discussed on 2020-05-09, stable images are now being built with the freedombox backport included to avoid users having to wait two unattended upgrades, fixing things like #1832 (closed). As released in buster, the freedombox package (eventually) pins itself to buster-backports.

Importance of disabled by default in bullseye:

• Debian default expectations, downgrades not an option
• User interface doesn't change over stable lifecycle
• Same experience whether installed immediately after release or 18 months later
• Fixes should be made available through proposed-updates

Benefit of two major exceptions: freedombox and matrix-synapse

• freedombox: important fixes for buster missed release
• simplified UI like hiding Advanced Apps
• matrix-synapse: ecosystem had big break in federation just as buster was released
• user shouldn't care about backports, developers in better position to judge compatibility concerns

Ultimately, all the points above boil down to conflating freedombox upstream with freedombox LTS (i.e. as released in stable). Many, many upstreams would like the ability to push updates to stable users, but freedombox markets itself as a pure Blend so should behave as normally as possible.