chore: mark GitPod configuration as legacy in dependabot

Add note to dependabot.yml documenting that GitPod configuration
(.gitpod.yml and .gitpod.Dockerfile) is now in legacy mode due to
migration to GitHub Codespaces.

GitPod configurations remain functional for users who prefer that
environment, but will not receive automated dependency updates going
forward. This aligns with the project's shift to Codespaces as the
primary cloud development environment (60h/month vs GitPod's 10h/month).

Author: gounthar

.github/dependabot.yml

@@ -1,5 +1,9 @@
 # Please see the documentation for all configuration options:
 # https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+#
+# Note: GitPod configuration (.gitpod.yml and .gitpod.Dockerfile) is in legacy mode
+# as we've migrated to GitHub Codespaces. GitPod configs will not receive dependency
+# updates but remain functional for users who prefer GitPod.
 
 version: 2
 # Enable version updates for GitHub Actions workflows