Mergify: switch from strict mode to a merge queue

[RyanGlScott]

Previously, we were using Mergify's smart mode to serialize the PRs that Mergify merges. This mode has been deprecated, however, and indeed, there is a smart mode brownout today to force Mergify users to switch over: https://blog.mergify.com/strict-mode-deprecation/ This PR does just that, based on the migration strategy suggested in that link.

[RyanGlScott]

Ugh. Despite the fact that this PR changes the Mergify configuration from an invalid configuration to a valid one, Mergify still refuses to consider it: https://github.com/GaloisInc/saw-script/pull/1535/checks?check_run_id=4435257575

I'm not quite sure what to make of this.

[RyanGlScott]

Now that Mergify's brownout is over, I can actually see what Mergify thinks of the configuration change:

Configuration changed. This pull request must be merged manually — 4 potential rules

‼️Action Required ‼

The configuration uses the deprecated strict mode of the merge action.
A brownout is planned for the whole December 6th, 2021 day.
This option will be removed on January 10th, 2022.
For more information: https://blog.mergify.com/strict-mode-deprecation/

⚠️The configuration has been changed, queue and merge actions are ignored. ⚠

Confusingly, it acknowledges that the configuration has changed while still warning about the use of strict mode, which is the very thing that this PR changes. More importantly, it claims that this must be merged manually due to the configuration change. I guess that's what we'll have to do, then. Any objections?

[kquick]

Looks good.