Unable to export a project with `Encoding::UndefinedConversionError`

A customer is unable to export a project on GitLab.com, they receive the error bellow when they try to do that. There are ~300 such events in Sentry for the past month so it likely to affect more users.

See this internal ticket for more details: https://gitlab.zendesk.com/agent/tickets/254125

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2960518/?referrer=gitlab_plugin

Encoding::UndefinedConversionError: "\x93" from ASCII-8BIT to UTF-8
  lib/gitlab/import_export/json/streaming_serializer.rb:77:in `block (3 levels) in serialize_many_relations'
    items << Raw.new(record.to_json(options))
  lib/gitlab/import_export/json/streaming_serializer.rb:76:in `block (2 levels) in serialize_many_relations'
    batch.each do |record|
  lib/gitlab/import_export/json/streaming_serializer.rb:103:in `block in batch'
    yield batch.reorder(batch.klass.primary_key)
  lib/gitlab/import_export/json/streaming_serializer.rb:98:in `batch'
    relation.in_batches(**opts) do |batch| # rubocop:disable Cop/InBatches
  lib/gitlab/import_export/json/streaming_serializer.rb:73:in `block in serialize_many_relations'
    batch(records, key) do |batch|
...
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