fix: update Dependabot path for gitsubmodule (#452)

[lubianat]

Related to #449, fixes #452

This assumes that we want Dependabot to track all submodules (not only dev, but also previous versions).

It will look into .gitmodules for the updates (which is in /, and not in /specifications/dev).

My experience with Dependabot is limited, though, so I might be overlooking something.

[github-actions]

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[jo-mueller]

My original idea was to keep the dependabot pointed only at the dev submodule by choice, because it's the only continuously developing submodule and I wouldn't expect any changes on the other branches in the future.

That being said, it would be cleaner if we put out releases from the 0.1...0.5 branches and then point the submodules to the releases rather than just the branches.

[lubianat]

I see! We can exclude the others, but I imahine that if eventually we change an older branch (say, to fix a broken link or similar), it may be good for the dependabot to trigger a PR here.

If they are not changed, then dependsbot will just stay silent, right?

Are there scenarios in which we would not want an update to be propagated here?

But either way works for me

[joshmoore]

On detecting changes to any of the branches, I take this as a plus if only so we know if a submodule was accidentally reverted due to local state.