arrow

GH-36056: [CI] Enable Dependabot for GitHub Actions (#36194)

### Rationale for this change

We can update GitHub Actions (semi) automatically by enabling Dependabot.

### What changes are included in this PR?

Add Dependabot configuration only for GitHub Actions.

### Are these changes tested?

No. Sorry. I want to test this by merging this to apache/arrow.

### Are there any user-facing changes?

No.
* Closes: #36056

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>