🎨 Renovate: Update gristlabs/grist Docker tag to v1.7.15

[SpiritLooper]

This PR contains the following updates:

Package	Update	Change
gristlabs/grist	patch	1.7.14 → 1.7.15

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This PR has been generated by Mend Renovate.

[github-actions]

Here is the diff to main:

--- k8s/apps/services Kustomization: flux-system/services HelmRelease: services/grist

+++ k8s/apps/services Kustomization: flux-system/services HelmRelease: services/grist

@@ -60,7 +60,7 @@

       envFrom:
       - secretRef:
           name: grist-creds
-      image: gristlabs/grist:1.7.14
+      image: gristlabs/grist:1.7.15
       ingress:
         domain: grist.egghead.infrao.top
       name: grist