This is the successor to #3665 (and #3563 and #3075). We should update our targetSdkVersion to 30, meaning Android 11.

Google Play has set a deadline of 2021-11-01 for this update, consistent with the pattern of the last few years.
(Based on the last few years' experience, Google Play will probably set a deadline for this of late 2021. 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.

One known blocker:

• Fix image capture + image upload to work with Android "scoped storage". #4283. The library we use for taking a photo to send via Zulip, or picking an existing image on the device to send via Zulip, hasn't yet adapted to "scoped storage", which arrived with Android 10. We've taken an opt-out for that, but the opt-out stops being available with Android 11.

Apart from that issue, we could do this any time. It'd be nice to do it well before the deadline -- or at least to learn about any other, unknown blockers.