bucket update shard-group-duration will change retention time

When trying to manually update the shard-group-duration, it automatically changes the retention time for the bucket aswell.

__Steps to reproduce:__

root@7397b17e9272:/# influx bucket update -i 0d217f1db07dc47e -r 60d
ID Name Retention Shard group duration Organization ID Schema Type
0d217f1db07dc47e test 1440h0m0s 1h0m0s fea73af8e7e51de1 implicit

root@7397b17e9272:/# influx bucket update -i 0d217f1db07dc47e --shard-group-duration 24h
ID Name Retention Shard group duration Organization ID Schema Type
0d217f1db07dc47e test 720h0m0s 24h0m0s fea73af8e7e51de1 implicit

I just created a test bucket for this purpose, and set the retention time to 60 days initially, but when I manually updated the shard-group-duration it changes the retention time. It seems the retention time will always be set to 30 days when changing the shard-group-duration

Start at the `influx bucket update` entry point and reproduce the two commands shown in the issue, first setting retention to 60 days and then changing `--shard-group-duration` to 24h. Done means changing the shard-group duration leaves the bucket retention at 60 days instead of resetting it to 30 days.

Written by the indexing model from the issue text.