Fix branch detection for snyk job since creation of 8.x branch

[jsvd]

The creation of the 8.x branch and moving main to 9.0.0 caused the report.sh script for the snyk buildkite job incorrectly detect branches, and needs to be adjusted:

  | Checking out 8.16 branch.
  | error: pathspec '8.16' did not match any file(s) known to git
  | 8.16 branch doesn't exist.
  | HEAD is now at a426e1b7f Known issue for 8.15.1 related to env vars references (#16455) (#16470)
  | Checking out 9.0 branch.
  | error: pathspec '9.0' did not match any file(s) known to git
  | 9.0 branch doesn't exist.