Changes

fix(nextgen): refresh exact image tag lookup branches (#4446)

## Summary
- update `scripts/ops/nextgen/get-next-gen-exact-image-tags.sh` to use
the newly cut next-gen release branch `release-nextgen-202603` for the
core next-gen repos
- switch the TiKV/CSE trunk lookup from `dedicated-next-gen` to
`cloud-engine-next-gen`
- sync `scripts/ops/nextgen/README.md` with the current script behavior
and covered registries/tags

## Validation
- `bash -n scripts/ops/nextgen/get-next-gen-exact-image-tags.sh`
- `PATH="$PWD/../.cache/bin:$PATH" shellcheck -S error
scripts/ops/nextgen/get-next-gen-exact-image-tags.sh`
- `git diff --check`

## Notes
- I did not run the script end-to-end against the registry in this
workspace because `gcloud` is not installed here, so the registry login
step cannot be exercised locally.
 (commit: 02c2b1d)