Follow 4.0 channel in juju-db snap

[timClicks]

Description of change

Juju requires stability over time. Switching to the 4.0 channel minimises the chances that we'll encounter breaking API changes.

QA steps

JUJU_DEV_FEATURE_FLAGS=mongodb-snap juju bootstrap localhost s4

Creating Juju controller "s4" on localhost/localhost
...
Launching controller instance(s) on localhost/localhost...
 - juju-cba768-0 (arch=amd64)
...
Initial model "default" added

lxc exec juju-cba768-0 snap list juju-db

Name     Version  Rev  Tracking  Publisher  Notes
juju-db  4.0.9    16   4.0       juju-qa    -

From this output, we can determine that we're tracking the 4.0 channel.

Documentation changes

None.

Bug reference

• https://bugs.launchpad.net/juju/+bug/1826938

[timClicks]

$$merge$$