Update actions/cache

The actions/cache lib is removing most versions on Feb 1/25
https://github.com/actions/cache/releases/tag/v4.2.0

We're fine with just specifying the major version, as we've never had
any issues with minor version changes.

.github/workflows/push.yml

@@ -6,16 +6,16 @@ jobs:
 
     strategy:
       matrix:
-        otp: ['25.2.3']
-        elixir: ['1.14.3']
+        otp: ["25.2.3"]
+        elixir: ["1.14.3"]
 
     env:
       MIX_ENV: test
     steps:
       - uses: actions/checkout@v3
 
       - name: Cache dialyzer plts
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: priv/plts
           key: ${{runner.os}}-${{matrix.otp}}-${{matrix.elixir}}-plts