.github/workflows/config-diff-post-comment.js: improve handling of GH comment

.github/workflows/config-diff-post-comment.js: improve handling of GH comment

1. When no configuration changes are detected, delete the outdated
   configuration diff Github comment. This ensures that the PR does not
   have any misleading information about configuration changes.
2. Configuration changes might change with every push event, update the
   old configuration diff comment with the new configDiff that was
   calculated in the present run.

Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>