And update Flutter's supporting libraries to match.

When building for iOS (`flutter run`), I got this output:

  ios/Runner/AppDelegate.swift uses the deprecated
    @UIApplicationMain attribute, updating.

And sure enough, this change was made, automatically. Sure; might as
well commit it. The following seems to be the explanation for it:
  https://github.com/apple/swift-evolution/blob/43b894b0c/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md

Actually, I have a hunch that this will be enough for zulip#612,
'"Missing API declaration" warning from Apple'. As that issue says,
the only category of "required reason APIs" that Apple is alerting
about is "user defaults APIs". There's just one API in this
category: `NSUserDefaults`. Using Step 4 in these instructions from
a Flutter maintainer --
  flutter/flutter#145269 (comment)
-- I found that firebase_messaging and flutter_local_notifications,
but no other dependencies, have calls to NSUserDefaults without
declaring reasons in this category in a "privacy manifest" file.

Moreover, unlike in zulip-mobile, our Podfile calls
`use_frameworks!`, which should mean we don't have to manually copy
anything from our dependencies into a privacy manifest file of our
own. See zulip/zulip-mobile#5859 for a link to a mention in the
Flutter project about `use_frameworks!`.

So I'm hoping that the following upgrades, made here, will address
Apple's alerts:

- firebase_messaging to v14.9.0, in which that dependency gets a
  privacy manifest that declares a reason for using "user defaults
  APIs":
    https://github.com/firebase/flutterfire/releases/tag/firebase_messaging-v14.9.0

- flutter_local_notifications to v17.1.0. In v17.0.1, a "user
  defaults APIs" reason declaration was added to this dependency's
  privacy manifest:
    https://github.com/MaikuB/flutter_local_notifications/releases

Fixes: zulip#612

This is the result of `tools/upgrade pod-major`, and some small
adjustments for a breaking API change in share_plus.

Changelogs:
  https://pub.dev/packages/package_info_plus/changelog
  https://pub.dev/packages/share_plus/changelog

Tested on my iPhone and on the office Android device. In particular,
I checked that sharing message content via the message action sheet
still worked, and I checked that the "About Zulip" page still shows
the "App version" field correctly.