Update Ruby, Rails and dependencies

See merge request !5

It removes the ruby_dep dependency that doesn't work with Ruby 3

Address puma CVE and update bundle

See merge request !4

https://github.com/advisories/GHSA-7xx3-m584-x994

It fixes CVEs:
https://www.ruby-lang.org/en/news/2019/10/01/ruby-2-6-5-released/

* Update Rails from 5.2.3 to 6.0.0

* Run bundle exec rails app:update

* Install webpack

bundle exec rails webpacker:install

* Run migrations

* Update puma

* Add sass-rails gem

* Remove unused gems

* Forgot to add this gem

* Add webpacker config

Mainly to fix the nokogiri CVE:
https://nvd.nist.gov/vuln/detail/CVE-2019-5477

Make sure it detects the IBAN correctly and it returns banking_fee
instead of vpn

Update Ruby and Rails