update target sdk version to 31

[pt2121]

As mentioned in https://github.com/grote/Transportr/issues/852#issuecomment-1551918703, Google Play new requires the target SDK to be at least 31. Also see https://support.google.com/googleplay/android-developer/answer/11926878.

I went through the doc here & here, and made sure that we updated everything that we should - fortunately it's not that much. From what I see, we don't use any foreground service, notif, BroadcastReceiver so it seems like a smooth update :crossed_fingers:

[cla-bot[bot]]

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[pt2121]

@cla-bot I accept the CLA.

[Altonss]

Why not target level 33 directly? Is this significantly more work? I looked here https://developer.android.com/google/play/requirements/target-sdk and found that starting of August, 33 will be required by PlayStore

[pt2121]

That's an option. I think it'd be safer to make minimum incremental changes. The code hasn't been updated for awhile and it's prudent to start with a release with small changes.

[cla-bot[bot]]

Thank you for your pull request and welcome to our community! We require contributors to sign our Contributor License Agreement, and we don't seem to have the user @pt2121 on file. In order for your code to get reviewed and merged, please explicitly state that you accept the agreement. Alternatively, you can add a commit that adds yourself to https://github.com/grote/Transportr/blob/master/.clabot