Drop support for 1.9.3
Drop support for 1.9.3

This discussion[1] reminded me that we still have to support 1.9.3 in
this repo.

I propose we drop that, because:

- Support for this ruby version ended almost a year ago[2]
- No apps on GOV.UK are currently on 1.9.3[3]
- Our dependencies start to drop support for 1.9.3[4] [5]
- 2.X has cool new features which we use often in other projects
(keyword args, `%i[]`)

[1]
https://github.com/alphagov/gds-api-adapters/pull/429/files#r52297113
[2]
https://www.ruby-lang.org/en/news/2014/01/10/ruby-1-9-3-will-end-on-2015
/
[3]
https://docs.google.com/spreadsheets/d/1FJmr39c9eXgpA-qHUU6GAbbJrnenc0P7
JcyY2NB9PgU/edit#gid=1480786499
[4] https://github.com/alphagov/gds-api-adapters/pull/363
[5] https://github.com/alphagov/gds-api-adapters/pull/387

alphagov/gds-api-adapters