Rails 6 introduces retries per-exception, instead of a global count of
retries. Because ActiveJob 5.2 doesn't serialize the execution count
per-exception, when ActiveJob 6.0 picks up an "old" job it can't know
the exception count in the new format.

This can also be an issue if AJ 6.0 serializes a new job with
exception_executions which is later picked up by AJ 5.2, which would
clear exception_executions (since it has no knowledge of it).

Previously we handled this by resetting exception_executions, if it
wasn't defined on a job, which could result in the worst case retrying
the job 2x the times we should.

This commit changes how we handle loading a legacy job: instead of
resetting exception_executions, we instead will always use the global
executions count.

This way, jobs which only have one retry_on (and didn't have a behaviour
change in AJ 6) are backwards-and-forwards-compatible with counts
respected exactly.

Jobs with multiple retry_on will revert to the AJ5.2 behaviour if they
were ever run under AJ5.2.