Use dependabot to upgrade gh-actions versions

[adinauer]

Description

We should use dependabot to track gh-actions version changes and update them automatically. Prior art: https://github.com/getsentry/sentry-react-native/blob/main/.github/dependabot.yml

[philipphofmann]

@adinauer, are you referring to warnings similar to

We also have them on Cocoa, see https://github.com/getsentry/sentry-cocoa/actions/runs/3248326856 or https://github.com/getsentry/sentry-cocoa/actions/runs/3262681695.

[markushi]

@philipphofmann yes, those are the ones we see for Java too.

[markushi]

Revisit this once the github-action has been updated.

[romtsn]

We should use dependabot so it automatically updates the gh-actions whenever there are new releases.

[philipphofmann]

I think you only need to add

- package-ecosystem: "github-actions"
  directory: "/"
  schedule:
      interval: weekly

to the dependabot.yml.

We already have this for Cocoa.