chore(github): add skip-changelog label to GitHub actions dependabot PRs

[lemeurherve]

Having those PRs in release changelogs is not really useful for users.

Refs:

• https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#labels--
• chore: add skip-changelog label to dependabot PRs related to GHA docker-ssh-agent#351

Testing done

N/A

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[timja]

A clearer PR title would be:

add skip-changelog label to GitHub actions dependabot PRs