[v3] Migrations backwards compatibility

[ewjoachim]

According to our own doc, migrations named with version X are the migrations you can apply once you run on version X. According to our doc, people on v2.13 wishing to upgrade without disruption would:

• Change the code to v3.0
• Apply the v3.0 migrations
• Change the code to v3.1
• Apply the v3.1 migrations All the while their instance is running and processing jobs.

Compatibility matrix:

Code ➡️ ⬇️ Schema	v2.13	v3.0	v3.1
v2.13	✅	✅	❌
v3.0	❌	✅	✅
v3.1	❌	❌	✅

❌ means we don't have to make it compatible, but if it happens to be, it's not a problem, of course

We need to decide what we put in v3.0 and v3.1

Note: the change of status = aborting -> abort_requested might need a couple of trigger just for the time of the dual compatibility (adding the `abort_requested column and update its value based on the status, then add a trigger that synchronizes the 2 columns, and in the next release, delete the triggers and the column)