deps: Upgrade source_gen to fix CI

[chrisbobbe]

Fixes #2000.

[gnprice]

Thanks!

Looks like CI fails with this change:

Running pigeon...
Error: there were changes to pigeons:
 M android/app/src/main/kotlin/com/zulip/flutter/AndroidIntents.g.kt
 M android/app/src/main/kotlin/com/zulip/flutter/AndroidNotifications.g.kt
 M ios/Runner/Notifications.g.swift
 M lib/host/android_intents.g.dart
 M lib/host/android_notifications.g.dart
 M lib/host/notifications.g.dart

Fixing that with tools/check pigeon --fix --all, then I'll merge.