Stop using 's' as indicator that a method returns a list

[TiberiuGC]

Description

We are having a couple of methods that return either some instance or a list of those instances, which are very similar in name. In fact, the one that returns a list ends in an additional s (e.g. DescribeNodeGroupStack and DescribeNodeGroupStacks). This means that using and testing these functions can get error prone in particular scenarios (closes #5219). We've decided to replace the s with List for such cases.

Checklist

• [ ] Added tests that cover your change (if possible)
• [ ] Added/modified documentation as required (such as the README.md, or the userdocs directory)
• [ ] Manually tested
• [ ] Made sure the title of the PR is a good description that can go into the release notes
• [x] (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! :exploding_head:

• [ ] Backfilled missing tests for code in same general area :tada:
• [ ] Refactored something and made the world a better place :star2: