Build: Group dependabot PRs updating GitHub Actions

Build: Group dependabot PRs updating GitHub Actions

We have monthly automatic dependabot PRs for GitHub Actions. Unfortunately,
as of now we get a separate PR for every dependency which is a bit spammy
compared to regular commits updating source.

Thankfully, there's now a way to tell dependabot to submit a single PR per
a defined group. This change defines a single group to have a single dependabot
PR for all action updates.

Closes gh-5503

(cherry picked from commit 3cac1465b4b5539bb679a517fbb52e5419c1866e)