Disable dependabot version updates
Disable dependabot version updates

Update `dependabot.yml` to only raise PR for security versions, and not for version updates, as per documentation here:

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#open-pull-requests-limit

This is intended to reduce the amount of noisy PRs we get, so we can bump the gem versions (if convenient) at our own pace.

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