[ML] 'finished_time' still reported after job has been reopened

[dolaru]

Description

Found in 6.3.2

Currently, when a job closes, the finished_time key is created/updated in the job config and it's value represents the timestamp of when the job was last closed.

If the user reopens the job, we still show the last finished_time until the job closes again. This is misleading, as there shouldn't be a finished_time reported while a job is still open.

Steps to reproduce

1. Create a job
2. Open the job
3. Close the job
4. Re-open the job
5. Notice that finished_time is still being reported and the value coincides with the time at which the job was last closed.

[elasticmachine]

Pinging @elastic/ml-core

[benwtrent]

#32755 fixes this issue