[2.x oss] Bucket retention policy doesn't change back to "never"

Hi, when I want to change bucket with a set retention time to a bucket with no retention policy it won't change (that's for the UI). [The documentation](https://docs.influxdata.com/influxdb/v2.1/organizations/buckets/update-bucket/) also doesn't show a solution for the CLI.

__Steps to reproduce:__
1. Create a bucket with a retention time of e.g. 7 days in the UI
2. Change the bucket's retention time to "Never"
3. Save Changes.

__Expected behavior:__
The retention time changes to "Forever", like in the case of buckets that are created without a retention time.

__Actual behavior:__
Nothing happens, the previous retention time remains active.

__Environment info:__
InfluxDB 2.0.8
Server: e91d418
Frontend: cb0a87e

Start by reproducing the retention change in the UI using the steps in the issue, then trace the bucket update path and compare it with the CLI guidance in the linked documentation. Done means changing a bucket from a seven-day retention period to “Never” persists and displays “Forever” after saving.

Written by the indexing model from the issue text.