[TECH DEBT] Update os.listdir to using os.walk for Git Pillar data traversal

[dmurphy18]

Description of the tech debt to be addressed, include links and screenshots

Need to implement os.walk for handling traversal of Git Pillar branch names with a '/' slash in the Git branch name,

See Issue #65467 (comment), and PR #66954

Was about to try this approach after proving it's possibility in a test program, on the Salt 3006.x branch when the PR was merged, given it had passed all tests, and was ready for merger, and marked for the 3006.10 milestone, and that milestone is close to release.

Hence tech-debt to improve as suggested in the Issue link above

Versions Report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

[dmurphy18]

Closing since PR #67723 is merged.