fix(codeblock): Add border radius

[mejo-]

Contributes to: #6152

🖼️ Screenshots

🏚️ 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

[mejo-]

/backport to stable30

[mejo-]

/backport to stable29

[backportbot]

The backport to stable29 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable29
git pull origin stable29

# Create the new backport branch
git checkout -b backport/6453/stable29

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick c8f8f259

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/6453/stable29

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[backportbot]

The backport to stable30 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable30
git pull origin stable30

# Create the new backport branch
git checkout -b backport/6453/stable30

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick c8f8f259

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/6453/stable30

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[mejo-]

Manual backport to stable30: #6456