Require Buster or newer for running our test suite

Supporting running our test suite on Stretch, instead of ≥ Buster, blocks:

• #15450
• #15831 (closed)
• #15330
• #17031 (closed)
• dropping some Stretch backwards compatibility code in our test suite

Additionally, this would allow me to stop maintaining a custom QEMU package for Stretch.

We need to:

• Figure out, together with developers and sysadmins, what a suitable timeline could be → we can stop supporting Stretch, no earlier than 2020-11-17
• Upgrade Jenkins isotesters (sysadmin#17731 (closed))
• Update run_test_suite to reflect this requirement
• Update contributors doc to reflect this requirement