Changes

ci(pipelines): replace internal mirror URL for ticdc GCP jobs (#4295)

## Summary

This PR replaces the internal mirror URL
`hub-zot.pingcap.net/mirrors/internal` with
`us-docker.pkg.dev/pingcap-testing-account/internal` in TiCDC pipeline
configurations.

## Context

The TiCDC integration test jobs run on GCP environment, but
`hub-zot.pingcap.net` is an internal network address that is not
accessible from GCP. This causes download failures when fetching TiDB
components for `pingcap-inc` organization PRs.

## Changes

Updated 7 pipeline files in `pipelines/pingcap/ticdc/latest/`:
- `pull_cdc_kafka_integration_heavy/pipeline.groovy`
- `pull_cdc_kafka_integration_light/pipeline.groovy`
- `pull_cdc_mysql_integration_heavy/pipeline.groovy`
- `pull_cdc_mysql_integration_light/pipeline.groovy`
- `pull_cdc_pulsar_integration_light/pipeline.groovy`
- `pull_cdc_storage_integration_heavy/pipeline.groovy`
- `pull_cdc_storage_integration_light/pipeline.groovy`

## Impact

- For `pingcap-inc` organization PRs: now uses
`us-docker.pkg.dev/pingcap-testing-account/internal` (accessible from
GCP)
- For external contributor PRs: continues to use
`us-docker.pkg.dev/pingcap-testing-account/hub` (unchanged)
- No changes to other mirror configurations (e.g.,
`hub-zot.pingcap.net/mirrors/hub` comments remain as documentation)

## Testing

The change only affects the OCI artifact download location. The existing
CI tests should validate the functionality.
 (commit: 8e72629)