CI: Set up dependabot for GitHub Actions

[akihironitta]

What does this PR do?

The bot will make sure to update all GH actions within the repo.

Before submitting

• [x] Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• [x] Did you read the contributor guideline, Pull Request section?
• [x] Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes?
• [n/a] Did you write any new necessary tests? [not needed for typos/docs]
• [n/a] Did you verify new and existing tests pass locally with your changes?
• [n/a] If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• [x] Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov[bot]]

Codecov Report

Merging #1412 (edacb40) into master (2657315) will decrease coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
- Coverage   91.79%   91.75%   -0.04%     
==========================================
  Files         286      286              
  Lines       12861    12861              
==========================================
- Hits        11806    11801       -5     
- Misses       1055     1060       +5     

Flag	Coverage Δ
unittests	91.75% <ø> (-0.04%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/text/question_answering/collate.py	95.29% <0.00%> (-4.71%)	:arrow_down:
flash/text/question_answering/model.py	93.87% <0.00%> (-0.69%)	:arrow_down:

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