ci: Use GITHUB_OUTPUT envvar instead of set-output command (#240)

`save-state` and `set-output` commands used in GitHub Actions are
deprecated and [GitHub recommends using environment
files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `set-output` to `$GITHUB_OUTPUT`

Instructions for envvar usage from GitHub docs:


https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
       - name: Get Composer cache directory
         id: composer-cache
         run: |
-          echo "::set-output name=dir::$(composer config cache-files-dir)"
+          echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
       - uses: actions/cache@937d24475381cd9c75ae6db12cb4e79714b926ed # v2.1.7
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
@@ -82,7 +82,7 @@ jobs:
       - name: Get Composer Cache Directory
         id: composer-cache
         run: |
-          echo "::set-output name=dir::$(composer config cache-files-dir)"
+          echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
 
       - uses: actions/cache@9b0c1fce7a93df8e3bb8926b0d6e9d89e92f20a7 # v3.0.11
         with:
@@ -106,7 +106,7 @@ jobs:
       - name: Get Composer Cache Directory
         id: composer-cache
         run: |
-          echo "::set-output name=dir::$(composer config cache-files-dir)"
+          echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
       - uses: actions/cache@9b0c1fce7a93df8e3bb8926b0d6e9d89e92f20a7 # v3.0.11
         with:
           path: ${{ steps.composer-cache.outputs.dir }}