Fix scripts/cells/check_router_routes_sync.rb script to not use /-/raw

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The following discussion from !252485 (merged) should be addressed:

• @tkuah started a discussion:

  follow-up: Using the /-/raw is not advisable. It is subject to rate limits which is quite low for unauthenticated requests.

  So this job may fail a lot. Suggest switching to an authenticated API request. Good news is that we can use CI_JOB_TOKEN too.

  • Repository Files API (raw endpoint): https://docs.gitlab.com/api/repository_files/
  • CI_JOB_TOKEN cross-project auth: https://docs.gitlab.com/ci/jobs/ci_job_token/

cc @marcogreg