chore(deps): update actions/cache action to v5

Author: renovate[bot]

.github/workflows/swift-ci.yml

@@ -33,7 +33,7 @@ jobs:
       - name: Cache Swift toolchain (Linux)
         if: runner.os == 'Linux'
         id: cache-swift-linux
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: /opt/swift
           key: swift-6.3.1-ubuntu-24.04
@@ -55,7 +55,7 @@ jobs:
       - name: Cache Swift toolchain (macOS)
         if: runner.os == 'macOS'
         id: cache-swift-macos
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: /Library/Developer/Toolchains/swift-6.3.1-RELEASE.xctoolchain
           key: swift-6.3.1-macos-14
@@ -97,7 +97,7 @@ jobs:
       # --- Build + test ----------------------------------------------------
 
       - name: Cache Swift build artefacts
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: .build
           key: swift-build-${{ runner.os }}-${{ hashFiles('Package.resolved') }}