fix(deps): update module github.com/spf13/viper (v1.18.2 → v1.21.0)

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/ci.yaml

@@ -20,7 +20,7 @@ jobs:
           go-version-file: "go.mod"
 
       - name: Cache Go build
-        uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cache/go-build

go.mod

@@ -3,5 +3,5 @@ module github.com/ivuorinen/tsm
 go 1.24
 
 require (
-	github.com/spf13/viper v1.18.2
+	github.com/spf13/viper v1.21.0
 )