chore: update GitHub workflows to use pnpm

- Use pnpm/action-setup for pnpm installation
- Use actions/setup-node@v6
- Add pnpm cache configuration

Author: imsus

.github/workflows/lint.yml

@@ -30,7 +30,7 @@ jobs:
           version: "latest"
 
       - name: Setup Node
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: "latest"
           cache: "pnpm"

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
           coverage: xdebug
 
       - name: Setup Node
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: "22"
           cache: "pnpm"