Rename Import Uploader extension allowlist constant

The following discussion from !59976 (merged) should be addressed:

• @arturoherrero started a discussion:

  We are inheriting here but we should create another merge request to change this:

      EXTENSION_ALLOWLIST = %w[ndjson.gz].freeze

  https://docs.gitlab.com/ee/development/documentation/styleguide/#culturally-sensitive-language