chore(revert): Revert "ci: update protoc version from 25.3 to 33.2 in regenerate-all.yml" (#17232)

Reverts #17228

Protoc 25.3 is still in use here:
https://github.com/googleapis/librarian/blob/3ec07bb1adbad63953dbd741427f92f00fea2646/cmd/librarian/Dockerfile#L49

Author: parthea

.github/workflows/regenerate-all.yml

@@ -28,7 +28,7 @@ jobs:
       - name: Install protoc
         uses: arduino/setup-protoc@v3
         with:
-          version: "33.2"
+          version: "25.3"
 
       - name: Install pandoc
         run: |