remove previously deprecated attach/download sbom commands

[hectorj2f]

Summary

The attach/download SBOM commands have been declared as deprecated long time ago. I believe it is a good time to cleanup these not recommended actions.

Release Note

Remove deprecated attach/download SBOM.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.76%. Comparing base (2ef6022) to head (4fd4d2c).
Report is 219 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3891      +/-   ##
==========================================
- Coverage   40.10%   36.76%   -3.35%     
==========================================
  Files         155      202      +47     
  Lines       10044    12676    +2632     
==========================================
+ Hits         4028     4660     +632     
- Misses       5530     7435    +1905     
- Partials      486      581      +95     

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[Hayden-IO]

Will hold til v3

[vishal-chdhry]

Hi @hectorj2f, @haydentherapper,
Some of us are facing a problem while downloading SBOMs using the warnings mentioned in the command. See: ko-build/ko#1346
Is there a different way to download SBOMs

[Hayden-IO]

@hectorj2f @cpanato We're about push out a major release. Do we want to clean this up now? Can you also remind me (and the other maintainers @steiza and @cmurphy) what the context was for this? It was that SBOMs weren't signed so we deprecated support for attaching them?

We can now recommend using oras to attach an SBOM or any other related artifacts as referring artifacts.