chore(deps): update actions/cache action to v6

.github/workflows/jepsen-test-scheduled.yml

@@ -63,7 +63,7 @@ jobs:
       # ~100 jars at first launch. Keying on project.clj invalidates the
       # cache only when dependencies actually change.
       - name: Cache Maven and Leiningen artifacts
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             ~/.m2/repository

.github/workflows/jepsen-test.yml

@@ -35,7 +35,7 @@ jobs:
       # 429s during peak hours have been knocking the scheduled stress
       # run out, and the per-push run uses the same dependency set.
       - name: Cache Maven and Leiningen artifacts
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             ~/.m2/repository

.github/workflows/tla-check.yml

@@ -74,7 +74,7 @@ jobs:
       # whole point of the pin (codex P2 on PR #856 round 4).  A full
       # miss-and-redownload is correct here.
       - name: Cache tla2tools.jar
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: .cache/tla
           key: ${{ runner.os }}-tla-tools-${{ hashFiles('Makefile') }}