i18n-tasks | Manage translation and localization with static analysis | Internationalization library

First, as ThorTL67 noted in the comments, it is a good idea to update your Rails version incrementally, not in one big leap. That way, your dependency issues will be less complex.

Check what version of Bundler you are using by running bundle version. If that version is old, it might be that some dependencies are not correctly calculated, and it might help to update Bundler (gem update bundler).

Then to the steps you can take to update from 4.2 to 'some higher version'. The list of errors that you got shows the conflicts between gem dependencies. You can try and tackle these conflicts one by one.

SQLite does not work with Heroku as its disk based and Heroku uses an ephemeral file system.

SQLite runs in memory, and backs up its data store in files on disk. While this strategy works well for development, Heroku’s Cedar stack has an ephemeral filesystem. You can write to it, and you can read from it, but the contents will be cleared periodically. If you were to use SQLite on Heroku, you would lose your entire database at least once every 24 hours.

Even if Heroku’s disks were persistent running SQLite would still not be a good fit. Since SQLite does not run as a service, each dyno would run a separate running copy. Each of these copies need their own disk backed store. This would mean that each dyno powering your app would have a different set of data since the disks are not synchronized.

-Heroku Devcenter: SQLite on Heroku

Heroku provides Postgres as the free default database for rails which is as close to a recommendation as you can get.

If you are deploying to Postgres you should also be developing/testing on Postgres.

Differences between backing services mean that tiny incompatibilities crop up, causing code that worked and passed tests in development or staging to fail in production. These types of errors create friction that disincentivizes continuous deployment. The cost of this friction and the subsequent dampening of continuous deployment is extremely high when considered in aggregate over the lifetime of an application.
- https://12factor.net/dev-prod-parity

If you really want to stick with SQLite you need to configure the adapters properly: