Changes

fix(pipelines): fix and use OCI_TAG_* for next-gen pipeline artifacts (#3851)

This pull request updates several Jenkins pipeline scripts to improve
how OCI artifact tags are determined and used for various TiDB and TiCDC
integration tests. The main change is to switch from using hardcoded
branch-based tags to dynamically computed OCI tags based on the base
reference of the pull request, specifically targeting "next-gen"
releases. This enhances flexibility and ensures the correct artifacts
are fetched for both standard and "next-gen" release branches.

Key changes include:

**Dynamic OCI Tag Calculation:**
- Replaced hardcoded `TARGET_BRANCH_*` variables with `OCI_TAG_*`
variables that select tags based on whether the base ref matches
`release-nextgen-*`. If it does, the base ref itself is used; otherwise,
a default "next-gen" tag is applied (e.g., `master-next-gen`,
`dedicated-next-gen`). This logic is applied across all affected
pipeline scripts.
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**Artifact Download Script Invocation Updates:**
- Updated all invocations of `download_pingcap_oci_artifact.sh` to use
the new `OCI_TAG_*` variables instead of the old `TARGET_BRANCH_*`
variables, ensuring the correct artifacts are downloaded for the current
test context. This affects both heavy and light integration test
pipelines for JDBC, MySQL, Kafka, Pulsar, and Storage.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
 (commit: 2996bb9)