Fix v2 default licenses

What does this MR do and why?

Fix mutating concat when fetching license data from data objects. Mutating concat causes all licenses to be recorded as default_licenses which results in incorrect results.

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Before	After

How to set up and validate locally

1. Create a vendor/package_metadata/licenses/v2/pypi/0/0.ndjson file with contents of one line as above:

   {"name":"requests","lowest_version":"2.3.0","highest_version":"2.31.0","default_licenses":["Apache-1.0","Apache-2.0"],"other_licenses":[{"licenses":["ISC"],"versions":["0.10.2"]},{"licenses":["Apache-1.0"],"versions":["1.0.0"]},{"licenses":["unknown"],"versions":["0.13.2","0.13.5"]},{"licenses":["MIT"],"versions":["0.0.1"]}]}

2. bundle exec rails console

3. Run sync service

   # optionally disable other purl_types if you have data under vendor/package_metadata/licenses already
   lease = Gitlab::ExclusiveLease.new("foo", timeout: 5.minutes)
   lease.try_obtain
   PackageMetadata::SyncService.execute(data_type: 'licenses', lease: lease)

4. Check default licenses

   license_ids = PackageMetadata::Package.find_by(purl_type: 'pypi', name: 'requests').licenses[0]
   PackageMetadata::License.where(id: license_ids).pluck(:spdx_identifier)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #418114 (closed)