Create dependabot.yml to ensure all GitHub Actions are kept up to date

[Trainax]

Some workflows in this repository use deprecated Node 12 GitHub Actions. Some examples are:

• actions/checkout@v2 and actions/create-release@v1 in create-release
• actions/checkout@v2, actions/***[email protected] and slackapi/[email protected] in build-release

Those actions should be updated to their last version which uses Node 16 to avoid running old deprecated Node 12 Actions in Node 16 (for more information see here).

By configuring dependabot automatic PRs will be made to ensure all actions are kept up to date