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

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.
```

Start with the report.sh script used by the Snyk Buildkite job and inspect how it derives branch names after the 8.x branch was created and main moved to 9.0.0. Reproduce the reported checkout messages and verify that the job detects and checks out the intended branches without the pathspec errors.

Written by the indexing model from the issue text.