ci: bump datum-cloud/actions reusable workflows to v1.15.0

Update the shared CI workflows to the latest release:
- validate-kustomize.yaml: v1.6.1 -> v1.15.0
- publish-docker.yaml:      v1.14.0 -> v1.15.0
- publish-kustomize-bundle.yaml: v1.14.0 -> v1.15.0

Verified the callers' inputs remain compatible at v1.15.0: validate-kustomize
takes no inputs; publish-docker requires only image-name (supplied);
publish-kustomize-bundle requires bundle-name/bundle-path (supplied). No new
required inputs were introduced.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: scotwells

.github/workflows/publish.yaml

@@ -16,7 +16,7 @@ jobs:
       contents: read
       packages: write
       attestations: write
-    uses: datum-cloud/actions/.github/workflows/publish-docker.yaml@v1.14.0
+    uses: datum-cloud/actions/.github/workflows/publish-docker.yaml@v1.15.0
     with:
       image-name: network-services-operator
       platforms: linux/amd64,linux/arm64
@@ -27,7 +27,7 @@ jobs:
       id-token: write
       contents: read
       packages: write
-    uses: datum-cloud/actions/.github/workflows/publish-kustomize-bundle.yaml@v1.14.0
+    uses: datum-cloud/actions/.github/workflows/publish-kustomize-bundle.yaml@v1.15.0
     with:
       bundle-name: ghcr.io/datum-cloud/network-services-operator-kustomize
       bundle-path: config

.github/workflows/validate-kustomize.yaml

@@ -5,4 +5,4 @@ on:
 
 jobs:
   validate-kustomize:
-    uses: datum-cloud/actions/.github/workflows/validate-kustomize.yaml@v1.6.1
+    uses: datum-cloud/actions/.github/workflows/validate-kustomize.yaml@v1.15.0