Optional CI changes

** Why are these changes being introduced:

There are a few other regular maintenance changes that we should make to
our CI workflow:

- actions/checkout should move from v2 to v4

- Our use of a set-output command is going to be deprecated, and a new
  syntax is available (and has been adopted by the Bedrock project which
  we are following)

- PHP 8.4 has been released by that community, so we should be checking
  our network against that release

** Relevant ticket(s):

n/a for these changes, but I'm still grouping them under PW-125

** How does this address that need:

Each of the bullets above has been addressed:

- actions/checkout has been upgraded from v2 to v4

- The set-output syntax is removed in favor of the environment files,
  per https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

  Each of the above changes is made to match what the Bedrock project is
  doing.

- PHP 8.4 is added to our support matrix

  Bedrock has not yet added PHP 8.4 to their support matrix, but this
  feels like a justifiable change to make on our own.

** Document any side effects to this change:

None

Author: matt-bernhardt

.github/workflows/ci.yml

@@ -8,10 +8,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-version: ['8.1', '8.2', '8.3']
+        php-version: ['8.1', '8.2', '8.3', '8.4']
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v2
@@ -20,7 +20,7 @@ jobs:
 
     - name: Cache Composer dependencies
       id: composer-cache
-      run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+      run: echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
 
     - uses: actions/cache@v4
       with: