chore: disable GitPod Dependabot updates (legacy platform)

Add note that GitPod is legacy (free tier sunset) and we've migrated
to GitHub Codespaces. Disable Dependabot monitoring of GitPod image
updates by setting open-pull-requests-limit to 0.

Codespaces uses devcontainer.json which cannot be monitored by
Dependabot (not a Dockerfile). Manual updates as needed.

Author: gounthar

.github/dependabot.yml

@@ -2,6 +2,10 @@
 # https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
 
 version: 2
+# Note: GitPod support is legacy (free tier sunset). Migrated to GitHub Codespaces.
+# Codespaces uses devcontainer.json which Dependabot cannot monitor (not a Dockerfile).
+# Manual updates for mcr.microsoft.com/devcontainers/base:ubuntu-24.04 as needed.
+
 # Enable version updates for GitHub Actions workflows
 updates:
   - package-ecosystem: "github-actions"
@@ -62,4 +66,13 @@ updates:
     directory: "./dockerfiles/dotnet"
     schedule:
       interval: weekly
-    open-pull-requests-limit: 10  
+    open-pull-requests-limit: 10
+
+  # GitPod (legacy) - disabled, not actively maintained
+  - package-ecosystem: docker
+    directory: "./.gitpod"
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 0
+    ignore:
+      - dependency-name: "gitpod/workspace-full"