Explicitly add csv to Gemfile

We were getting a warning on staging and production environments:

```
app/models/local_census_records/import.rb:1: warning: csv was loaded
from the standard library, but will no longer be part of the default
gems starting from Ruby 3.4.0.

You can add csv to your Gemfile or gemspec to silence this warning
```

The reason we weren't getting this warning during development is that we
do have `csv` in our `Gemfile.lock`, but only in development
environments, since it's an indirect dependency of pronto. On production
environments, we don't install pronto or its dependencies, though.

We can reproduce the warning locally by temporarily removing the pronto
gems from the Gemfile, running `bundle install` and starting a rails
console.

Gemfile

@@ -17,6 +17,7 @@ gem "caxlsx", "~> 4.2.0"
 gem "caxlsx_rails", "~> 0.6.4"
 gem "ckeditor", "~> 4.3.0"
 gem "cocoon", "~> 1.2.15"
+gem "csv", "~> 3.3.5"
 gem "daemons", "~> 1.4.1"
 gem "dalli", "~> 3.2.8"
 gem "delayed_job_active_record", "~> 4.1.11"