skip 6.8 branch when triggering baseline-capture builds (#72706)

Co-authored-by: spalger <spalger@users.noreply.github.com>

.ci/Jenkinsfile_baseline_trigger

@@ -16,6 +16,12 @@ kibanaLibrary.load()
 
 withGithubCredentials {
  branches.each { branch ->
+ if (branch == '6.8') {
+ // skip 6.8, it is tracked but we don't need snapshots for it and haven't backported
+ // the baseline capture scripts to it.
+ return;
+ }
+
  stage(branch) {
  def commits = getCommits(branch, MAXIMUM_COMMITS_TO_CHECK, MAXIMUM_COMMITS_TO_BUILD)