Automatic version updates for github actions with dependabot

[iemejia]

Related Issues/PRs

#xxx

What changes are proposed in this pull request?

Add a dependabot file to update the versions of the Github Actions used for CI jobs

Briefly describe the changes included in this Pull Request.
This requires to create a dependabot.yml file and to enable dependabot in the settings of the repository

How is this patch tested?

I applied it in my fork and it proposed the correct updates (and a security alert).
https://github.com/iemejia/SynapseML/pulls

• I have written tests (not required for typo or doc fix) and confirmed the proposed feature/bug-fix/change works.

Does this PR change any dependencies?

• No. You can skip this section.
• Yes. Make sure the dependencies are resolved correctly, and list changes here.

Does this PR add a new feature? If so, have you added samples on website?

• No. You can skip this section.
• Yes. Make sure you have added samples following below steps.

1. Find the corresponding markdown file for your new feature in website/docs/documentation folder.
   Make sure you choose the correct class estimators/transformers and namespace.
2. Follow the pattern in markdown file and add another section for your new API, including pyspark, scala (and .NET potentially) samples.
3. Make sure the DocTable points to correct API link.
4. Navigate to website folder, and run yarn run start to make sure the website renders correctly.
5. Don't forget to add <!--pytest-codeblocks:cont--> before each python code blocks to enable auto-tests for python samples.
6. Make sure the WebsiteSamplesTests job pass in the pipeline.

AB#1922685

[github-actions]

Hey @iemejia 👋!
Thank you so much for contributing to our repository 🙌.
Someone from SynapseML Team will be reviewing this pull request soon.
We appreciate your patience and contributions 💯!

[mhamilton723]

Very cool!

[iemejia]

We can add automatic dependency updates for other ecosystems (scala, .net, etc) but given how error-prone are the dependency updates of Spark and friends it is probably better to be conservative, but let me know if you want me try a PR for that.

[iemejia]

Oups seems like sbt is not supported yet
dependabot/dependabot-core#352