Update github-actions to v6

Author: gardener-ci-robot

.github/actions/prepare-release/action.yaml

@@ -3,7 +3,7 @@ name: Prepare-Release
 runs:
   using: composite
   steps:
-    - uses: actions/setup-go@v5
+    - uses: actions/setup-go@v6
     - name: prepare-release
       shell: bash
       run: |

.github/workflows/build.yaml

@@ -51,7 +51,7 @@ jobs:
         with:
           commit-objects-artefact: ${{ needs.prepare.outputs.version-commit-artefact }}
           after-import: rebase
-      - uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5  # v5.5.0
+      - uses: actions/setup-go@44694675825211faa026b3c33043df3e48a5fa00  # v6.0.0
         with:
           go-version-file: 'go.mod'
       - name: build-diki
@@ -135,7 +135,7 @@ jobs:
     permissions:
       contents: read
     steps:
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version: '1.25'
       - uses: gardener/cc-utils/.github/actions/trusted-checkout@master