Update Ruby, Rails version. Also upgrade Heroku stack to 22

[carloslopes]

We currently use Ruby version ~> 2.6 and Rails version ~> 5.2.6.3. These versions are reaching their EOL this month, so we need to update them ASAP.

There's an opened PR (#793) that already did the Ruby version update, and now we need to implement the Rails version upgrade on it to achieve the goal.

[wrspada02]

Nowadays we need to update to LTS ruby and rails version. Also, it has to upgrade the Heroku stack to 22.