Changes

revert: rollback nextgen CSE trunk-branch switch (#4450)

## Summary
- revert the 17 pipeline-line changes introduced by `PingCAP-QE/ci#4443`
- restore `dedicated` / `dedicated-next-gen` defaults in the affected
nextgen TiKV-related pipelines
- keep the revert scoped to the original merged change on top of current
`main`

## Why
`#EE-CICD` requested rolling back `ci#4443`. I checked current `main`
first and confirmed the `#4443` edits had not been superseded by later
PRs on those files, so a direct revert is the smallest safe rollback.

## Validation
- `git diff --check origin/main...HEAD`
- `rg -n "cloud-engine(-next-gen)?" <affected files>` returns no matches
after the revert

## Reverts
- PingCAP-QE/ci#4443
- commit `702ea2e07a63aad651fb839fa3ec99c782b9f582`
 (commit: 4adb508)