Add Ubuntu 24.04 to datasource when upstream marks it as beta

[HonkingGoose]

Describe the proposed change(s).

• Add Ubuntu 24.04 to datasource/github-runners when actions/runner-images marks it as beta. The new runner should get the isStable:false property at first.
• Replace ubuntu-22.04 with ubuntu-24.04 in the readme.md file in the datasource/github-runners directory.

Relevant comment from upstream:

@TomKeur the announcement you pointed at is about GA, GA will be somewhere in August I think, but public beta is mid-May

Source for comment, and upstream issue to monitor:

• Add Ubuntu 24.04 Noble Numbat actions/runner-images#9691 (comment)

The files we need to edit are can be found in Renovate's github-runners folder.