Add support for script as a dict in update

[haynesgt]

Closes #1560

[miguelgrinberg]

Rebasing in preparation for merging, plus some minor changes to ensure compliance with the ES specification for scripted updates.

[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-1562-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 f4f6f138849d70de4099f54cccf376c2d4b2e540
# Push it to GitHub
git push --set-upstream origin backport-1562-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-1562-to-8.x.