[Dependabot] For GitHub Actions, group together all actions officially built by GitHub

[tdonohue]

Description

Improves our configuration of dependabot for GitHub Actions by doing the following:

• Automatically group all actions officially created by GitHub into a single PR.
• Only update actions officially created by GitHub when a new major version is released.
  • This is because we use a "rolling tag" (e.g. @v2) for officially GitHub actions. For instance @v2 means run the latest 2.x.x version...so, it auto-updates already. We only need to update our GitHub action when version 3 would be released.
  • NOTE: We do not use "rolling tag" versions for non-official actions, just for safety. So, this only impacts official actions from GitHub.

This PR should be ported to DSpace/DSpace backend repo as well. It only needs to be applied to the main branch though as that branch contains all dependabot rules.