Change default_priority default value to None

Currently, default priority is a [common service configuration option](https://bugwarrior.readthedocs.io/en/latest/common_configuration.html#common-service-configuration-options) which defaults to `M`. It is generally agreed that a better default would be `None` -- that is to assign no priority to tasks by default. [1](#426) [2](#780) [3](https://github.com/GothenburgBitFactory/bugwarrior/pull/1080#issuecomment-2566445796)

The challenge is in doing this gracefully. To quote myself:

> I agree that this would be a better default but I'm not sure there's a graceful way to make the change at this point. This would be a drastic disruption of the priority economy in existing databases in which users may have already adjusted their coefficients in line with bugwarrior's default...

However, I now think this is bothersome enough to be worth pursuing, even if it does take a long time and several release cycles. I propose the following:

- [ ] Add a [top level](https://bugwarrior.readthedocs.io/en/latest/common_configuration.html#main-section) default_priority setting with a default value of `M` which is the default value of the per-service default_priority options.
- [ ] Make this setting required. (In a future major release of bugwarrior. 3.0?)
- [ ] Make this setting optional with a default value of `None`. (In a subsequent major release of bugwarrior. 4.0?)