Changes

chore(pingcap/tidb): disable periodics_tidb_next_gen_smoke_test job (#3739)

This pull request disables the `periodics_tidb_next_gen_smoke_test`
Jenkins job and makes updates to the periodic CI configuration for TiDB.
The most important changes are grouped below:

**Jenkins Job Management:**

* The `periodics_tidb_next_gen_smoke_test` Jenkins job is now explicitly
disabled in its Groovy definition
(`jobs/pingcap/tidb/latest/periodics_tidb_next_gen_smoke_test.groovy`).

* The corresponding periodic job entry for
`periodics_tidb_next_gen_smoke_test` has been removed from the Prow
configuration (`prow-jobs/pingcap/tidb/latest-periodics.yaml`).
 (commit: fc4ab0a)