Target Android 14 (set targetSdkVersion to 34), by 2023-10 deadline

[gnprice]

This is the successor to #5453 (and an annual series of previous issues linked from there). We should update our targetSdkVersion to 34, meaning Android 14.

The deadline ~~is earlier this year than in some previous years: it's 2024-08-31. (Much like last year, we can request an extension to 2024-11-01.)~~ is 2024-10-31, after I requested an extension.

The important steps for this upgrade are:

1. Read about the potentially-breaking changes, and identify those that might affect us. (edit: Also the changes not tied to targetSdkVersion, because for Android 13 last year a change that should have been in the first doc wasn't, and bit us, and the latter doc had an item that would have raised our suspicions if we'd read that doc closely.)
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.