upgrade: remove last full scan from get jobs query

This removes the last full scan from the query used to find migration
jobs.

```
  • root
  │
  ├── • hash join
  │   │ equality: (job_id) = (id)
  │   │ right cols are key
  │   │
  │   ├── • render
  │   │   │
  │   │   └── • lookup join
  │   │       │ table: job_info@primary
  │   │       │ equality: (id, lookup_join_const_col_@16) = (job_id,info_key)
  │   │       │
  │   │       └── • render
  │   │           │
  │   │           └── • scan buffer
  │   │                 label: buffer 1 (running_migration_jobs)
  │   │
  │   └── • scan buffer
  │         label: buffer 1 (running_migration_jobs)
  │
  └── • subquery
      │ id: @s1
      │ original sql: SELECT id, status FROM system.jobs WHERE (status IN ('running', 'pending', 'cancel-requested', 'pause-requested', 'reverting', 'paused')) AND (job_type = 'MIGRATION')
      │ exec mode: all rows
      │
      └── • buffer
          │ label: buffer 1 (running_migration_jobs)
          │
          └── • filter
              │ filter: status IN ('cancel-requested', 'pause-requested', 'paused', 'pending', 'reverting', 'running')
              │
              └── • index join
                  │ table: jobs@primary
                  │
                  └── • scan
                        missing stats
                        table: jobs@jobs_job_type_idx
                        spans: [/'MIGRATION' - /'MIGRATION']
```

Note that if a migration job has more than 1 legacy_payload key, it
will cause an assertion failure error since the query will produce
multiple rows. We should not have multiple legacy_payload keys because
we delete any existing payload keys anytime we write a new payload
key.

Epic: none

Release note: None

Author: stevendanna

pkg/upgrade/upgrademanager/manager.go

@@ -792,28 +792,28 @@ FROM (
      WHERE status IN ` + jobs.NonTerminalStatusTupleString + `
 )
 WHERE ((pl->'migration')->'clusterVersion') = $1::JSONB`
+	// postJobInfoQuery avoids the crdb_internal.system_jobs table
+	// to avoid expensive full scans.
 	postJobInfoTableQuery = `
-WITH latestpayload AS (
-     SELECT job_id, value FROM system.job_info AS payload
-     WHERE info_key = 'legacy_payload'
-     ORDER BY written DESC
+WITH
+running_migration_jobs AS (
+    SELECT id, status
+    FROM system.jobs
+    WHERE status IN ` + jobs.NonTerminalStatusTupleString + `
+    AND job_type = 'MIGRATION'
+),
+payloads AS (
+    SELECT job_id, value
+    FROM system.job_info AS payload
+    WHERE info_key = 'legacy_payload'
+    AND job_id IN (SELECT id FROM running_migration_jobs)
+    ORDER BY written DESC
 )
-SELECT id, status
-FROM (
-     SELECT
-     distinct(id),
-     status,
-     crdb_internal.pb_to_json(
-	'cockroach.sql.jobs.jobspb.Payload',
-	payload.value,
-	false -- emit_defaults
-     ) AS pl
-     FROM system.jobs AS j
-     INNER JOIN latestpayload AS payload ON j.id = payload.job_id
-     WHERE j.status IN ` + jobs.NonTerminalStatusTupleString + `
-     AND j.job_type = 'MIGRATION'
-)
-WHERE ((pl->'migration')->'clusterVersion') = $1::JSONB`
+SELECT id, status FROM (
+    SELECT id, status, crdb_internal.pb_to_json('cockroach.sql.jobs.jobspb.Payload', payloads.value, false) AS pl
+    FROM running_migration_jobs AS j
+    INNER JOIN payloads ON j.id = payloads.job_id
+) WHERE ((pl->'migration')->'clusterVersion') = $1::JSONB`
 )
 
 func (m *Manager) getRunningMigrationJob(