Switch our cross-compile environments to Debian Buster

Our cross-builds currently use Debian Stretch. We should switch them to Debian Buster.

Trac:
Child Ticket(s): legacy/trac#31472 (moved), legacy/trac#31470 (moved), legacy/trac#31471 (moved)

added component::applications/tor browser in Legacy / Trac owner::tbb-team in Legacy / Trac priority::medium in Legacy / Trac resolution::duplicate in Legacy / Trac severity::normal in Legacy / Trac status::closed in Legacy / Trac tbb-rbm in Legacy / Trac type::task in Legacy / Trac labels

Trac:
Type: defect to task

Duplicate of legacy/trac#31127 (moved).

Trac:
Status: new to closed
Resolution: N/A to duplicate

closed

mentioned in issue legacy/trac#31470 (moved)

mentioned in issue legacy/trac#31471 (moved)

mentioned in issue legacy/trac#31472 (moved)

moved from legacy/trac#31469 (moved)

added Task label and removed 1 deleted label

removed 1 deleted label