Remove serverless client

[flobernd]

The serverless- and the stack- clients got merged into a single hybrid client.

[github-actions]

The backport to 8.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.x 8.x
# Navigate to the new working tree
cd .worktrees/backport-8.x
# Create a new branch
git switch --create backport-8451-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 5245d06c05a248770634aa484e7f6db7d1248e7e
# Push it to GitHub
git push --set-upstream origin backport-8451-to-8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.x

Then, create a pull request where the base branch is 8.x and the compare/head branch is backport-8451-to-8.x.