The scheduling TTL job of table created in v6.5 is particularly frequent after upgraded to 7.5 or later versions

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create table TTL table in 6.5
2. upgrade cluster to 7.5 or later versions
3. select histories in mysql.tidb_ttl_job_history to see the schedule history

2. What did you expect to see? (Required)

The TTL job should be scheduled every 1 hour.

3. What did you see instead (Required)

The history shows the schedule interval is much less than 1 hour.

4. What is your TiDB version? (Required)