Add Dependabot config to autoupdate GitHub action versions (#1764)

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### Summary

I noticed that GitHub workflows were throwing deprecation warnings due
to out-of-date action versions [[recent
example](https://github.com/Qiskit/qiskit-metapackage/actions/runs/5259141945)].

Rather than updating the versions manually this one time, this PR
introduces a Dependabot config that will trigger ongoing updates via PRs
created by Dependabot.

### Details and comments

If this merges, you can expect additional PRs shortly after from
Dependabot, for example, to update `actions/setup-python@v2` to v4.

I looked for a CHANGELOG file as mentioned in the PR template comments
but couldn't find one. I also searched for an "Unreleased" section in
all files but couldn't find one. Although this PR doesn't touch code,
please let me know how to comply with the requirements.

I read the CONTRIBUTING document and anticipate that I'll need to sign
the CLA after opening this PR.

---------

Signed-off-by: Kurt McKee <[email protected]>
Co-authored-by: Eric Arellano <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"