Remove bottom margin for better warning blocks display

[Jerome-Herbinet]

This CSS change removes the extra bottom padding (which is in fact a paragraph bottom margin). It makes text more compact, which is IMO relevant because warning blocks aim to be compact.

🏚️ Before

🏡 After

🏁 Checklist

• [ ] Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• [ ] Sign-off message is added to all commits
• [ ] Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• [ ] Documentation (README or documentation) has been updated or is not required

[Jerome-Herbinet]

@juliushaertl please check my new commit (old and new commit have been squashed)

[Jerome-Herbinet]

Testing by your side is welcome

[mejo-]

/backport to stable28

[github-actions[bot]]

Hello there, Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!