Use JSON directly in json to msgpack migration

[adamruzicka]

MultiJson provides a common interface over several JSON parsers. Sadly some of the parsers have subtle differences.

require 'json'
require 'oj'

example = '{"key": 1637704901.3995261}'
Oj.load(example)['key'].class # => BigDecimal
JSON.parse(example)['key'].class # => Float

[adamruzicka]

Rails 7 dropped support for the classic loader, considering tests are greenish in the zeitwerk PR, I'd say it is related. Let's pin rails below 7 for now and unpin it once the zeitwerk pr gets in