ci: enable Automatic Merge of Approved Pull Requests

[phracek]

Add automation that will merge approved PRs with passed CI checks automatically.

It uses the following GitHub Actions:

• gather-pull-request-metadata - to gather important data about PRs (used in following actions)
• pull-request-validator - to check if PR is approved and has passed CI checks and more
• auto-merge - to merge approved PRs
• issue-commentator - to comment on PR with status of PR and merge status

Here are some examples of use from the systemd source-git. Please notice the Pull Request validation and Auto Merge sections:

• https://github.com/redhat-plumbers/systemd-rhel9/pull/281#issuecomment-2165555171
• https://github.com/redhat-plumbers/systemd-rhel9/pull/277#issuecomment-2139901121
• https://github.com/jamacku/nginx-container/pull/5#issuecomment-2182743930 Similar PR is here: https://github.com/sclorg/nginx-container/pull/306 /cc @jamacku

[phracek]

I agree, but let's test it on two containers and later on we will decide whether we will use it in all repositories.