fix: spec.executor.instances is Optional, Support a flexible number of executors (cherry-pick #11877)#15202

gcp-cherry-pick-bot[bot] wants to merge 1 commit intorelease-2.6argoproj/argo-cd:release-2.6from

cherry-pick-609412-release-2.6argoproj/argo-cd:cherry-pick-609412-release-2.6Copy head branch name to clipboard