Update actions/cache from v2 to v4

** Why are these changes being introduced:

Github will be retiring actions/cache@v2 at the start of February 2025,
which this repository is currently configured to use.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/pw-125

** How does this address that need:

This updates our CI workflow to use actions/cache@v4, which is the
latest release. No other changes appear to be needed beyond a version
bump.

** Document any side effects to this change:

None

Author: matt-bernhardt

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
       id: composer-cache
       run: echo "::set-output name=dir::$(composer config cache-files-dir)"
 
-    - uses: actions/cache@v2
+    - uses: actions/cache@v4
       with:
         path: ${{ steps.composer-cache.outputs.dir }}
         key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}