Update timestamp format for unicorn_stderr.log

This MR updates the timestamp format, to follow the change introduced in gitlab-org/gitlab!24612 (merged)

Original issue: gitlab-org&2575

The new log line will be parsed correctly by the format regex: /^(?<type>[^\s]*), \[(?<time>[^\]]*) \#(?<pid>[^ ]*)\]\s+(?<typelong>[^\s]*)[\s:\-]*(?<message>(.*).*)/

e.g. the following line, E, [2020-02-07T15:11:15.529Z #49897] ERROR -- : reaped #<Process::Status: pid 8022 SIGKILL (signal 9)> worker=6 is parsed into groups as

E
2020-02-07T15:11:15.529Z
49897
ERROR
reaped #<Process::Status: pid 8022 SIGKILL (signal 9)> worker=6
reaped #<Process::Status: pid 8022 SIGKILL (signal 9)> worker=6