Update STACK_VERSION to 8.10-SNAPSHOT in validate-pr.yml (#2303)

Author: maxhniebergall

.github/workflows/validate-pr.yml

@@ -18,7 +18,7 @@ jobs:
     if: github.repository_owner == 'elastic' # this action will fail if executed from a fork
     runs-on: ubuntu-latest
     env:
-      STACK_VERSION: 8.9-SNAPSHOT
+      STACK_VERSION: 8.10-SNAPSHOT
 
     steps:
       - uses: actions/checkout@v2