ci: bump action versions, rm unused

.github/workflows/linux.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-20.04 # 20.04 glibc 2.31 - 22.10 glibc 2.36
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
           lfs: true
@@ -39,12 +39,9 @@ jobs:
           mkdir -pv ~/.cache/common-lisp
         shell: bash
 
-      - name: Get Current Month
-        id: current-month
-        run: echo "::set-output name=value::$(date -u '+%Y-%m')"
       - name: Cache SBCL Setup
         id: cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         env:
           cache-name: cache-sbcl-compressed-sdl
         with: