Automatic merge from submit-queue (batch tested with PRs 54460, 55258, 54858, 55506, 55510). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Fix namespace support in kubectl aging plugin

The example plugin's comments mentioned the namespace support
limitation, awaiting kubectl to implement environment variable
propagation. Now that 1.8 is out, the requirement is satisfied.

This commit adds use of the conveyed namespace information to the
plugin example.

Fixes #55255



**What this PR does / why we need it**:

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note

```