[tool] Use ^ for Dart SDK

[stuartmorgan-g]

The update-min-sdk command has been using explicitly ranges for the Dart SDK, because that used to be required. Current versions of pub no longer require that, and using ^ for Dart ranges is okay, so this updates the tooling to use that format in the future.

Also removes the special casing that changed the upper bound from 3 to 4, since we have passed the point where we are generating upper bounds less than 4 anyway.

To minimize churn, this doesn't update the existing pubspecs. We can incrementally adopt this going forward as we roll dependencies forward.

Fixes flutter/flutter#139806

[ditman]

The flutter constraint forbids an upper range, so we're going to end up with something like this:

sdk: ^3.2.0
flutter: ">=3.16.0"

(which had me doing a double take to grok why they're different :P)

[stuartmorgan-g]

Yes, but they were also different before (Dart requires upper bound, Flutter forbids it), so it's just differently different :)

[ditman]

Yes, but they were also different before (Dart requires upper bound, Flutter forbids it), so it's just differently different :)

Agreed, the flutter dependency now sticks out a little bit. Maybe we should propose a ∞ prefix so ∞3.16.0 means "anything after 3.16.0, with no upper bound" :P

[ditman]

The tree thinks that it needs to be red:

But in reality, build 605 passed.

The dashboard must have lost the memo.

(Manually squashing and merging this PR to get the dashboard unstuck.)

[ditman]

OK, it seems that the dashboard will retry 3x automatically without us having to use the github UI. In fact, it seems the dashboard is not picking up the retries triggered from the github UI?

If the above is true, the retries for LUCI tasks that are "red" should be triggered from the RERUN button of the dashboard, and not github. Like I did here for testing:

(Note how run 605 is not listed there)