internal/bg: Change log retention to round numbers

Created by: indradhanush

In this commit we change the log retention of old event logs in Postgres from 93 to 90 days.

And we change the log retention of old security event logs in Postgres from 186 to 180 days.

90 and 180 being round numbers are easy to remember and reference. Also 93 and 186 do not guarantee that they will be exactly 3 and 6 months depending on the time period in question. Example:

1. February
2. February but a leap year
3. July - August and December - January have back to back 31 days.

This is a follow up on #22025.