[CP -1.12] skip subresource in resource discovery

[reasonerjt]

Thank you for contributing to Velero!

Please add a summary of your change

Does your change fix a particular issue?

Fixes #(issue)

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Merging #6688 (1ae745c) into release-1.12 (0c0ccf9) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@              Coverage Diff              @@
##           release-1.12    #6688   +/-   ##
=============================================
  Coverage         60.36%   60.36%           
=============================================
  Files               242      242           
  Lines             25968    25979   +11     
=============================================
+ Hits              15676    15683    +7     
- Misses             9190     9194    +4     
  Partials           1102     1102           

Files Changed	Coverage Δ
pkg/discovery/helper.go	69.00% <66.66%> (-0.37%)	⬇️