This is the successor to #4284 (and #3665 and #3563 and #3075). We should update our targetSdkVersion to 31, meaning Android 12.

Based on the last few years' experience, Google Play will probably set a deadline for this of late 2022. It's been the first business day of November in each of the last few years.

The important steps for this upgrade are:

1. Read about the potentially-breaking changes, and identify those that might affect us.
2. Make a WIP change to targetSdkVersion.
3. Test the WIP change thoroughly, especially in the areas highlighted in step 1. Fix things as needed, and repeat.

We aren't aware of any blockers to this, so we can try doing it any time. It'd be nice to do it well before the deadline -- or at least to go far enough to learn about any unknown blockers.