Replace deprecated ::set-output with GITHUB_OUTPUT

GitHub deprecated the ::set-output workflow command in 2022 and the
runners now emit warnings on every CI run. Switch to writing the
'name=value' line into the file pointed to by \$GITHUB_OUTPUT, which is
the documented modern equivalent. The downstream cache step already
references steps.composer-cache.outputs.dir, no other change needed.

.github/workflows/tests.yaml

@@ -27,7 +27,7 @@ jobs:
 
       - name: Set composer cache directory
         id: composer-cache
-        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+        run: echo "dir=$(composer config cache-files-dir)" >> "$GITHUB_OUTPUT"
 
       - name: Restore composer from cache
         uses: actions/cache@v4