feat: added merge groups

[mfranzke]

Proposed changes

according to https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions

and setting up activating a new ruleset for merge groups, as documented: https://github.com/db-ux-design-system/core-web/settings/rules/3944111

We should additionally have a look at https://github.com/googleapis/release-please as well.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (fix on existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

[github-actions]

🔭🐙🐈 Test this branch here: https://db-ux-design-system.github.io/core-web/review/feat-added-merge-groups

[Copilot]

Pull Request Overview

This PR introduces support for triggering GitHub Actions workflows on merge group events as part of the new merge groups feature.

• Added merge_group event triggers to pull-request, pull-request-opened, and default workflow definitions.
• Configured branch filtering for merge group events in pull-request and pull-request-opened workflows.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
.github/workflows/pull-request.yml	Adds merge_group trigger with branch filtering for PR events
.github/workflows/pull-request-opened.yml	Adds merge_group trigger with branch filtering for opened PRs
.github/workflows/default.yml	Adds merge_group event trigger but without branch filtering

[Copilot]

The merge_group event trigger is added without branch filtering, unlike in the other workflows. Consider adding a branch specification (e.g., branches: ["main"]) to ensure consistency.

Suggested change

	merge_group:
	merge_group:
	branches:
	- "main"