Let dependabot make as many action PRs as it needs

[jsoref]

Currently package-ecosystem: "gomod" has no PR limit, but actions has the default limit of 5: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#open-pull-requests-limit

This should explain why the other actions didn't have open PRs (see #122).