Changes

fix(nextgen): rename next-gen tags to nextgen (#4556)

## Summary
- rename trunk nextgen image tag lookups from `*-next-gen` to
`*-nextgen` in the nextgen image tag helper script
- update nextgen pipeline defaults to use `master-nextgen` and
`cloud-engine-nextgen`
- align script wording from `next-gen` to `nextgen` / `TiDB X`

## Test
- not run (CI config and helper script changes only)
 (commit: d5f1c15)