[Transport] Bump to v8.6.0

[Anaethelion]

This brings a fix in the form of an updatable connection pool fixing a deadlock in transport.
See elastic/elastic-transport-go#20

[github-actions]

The backport to 8.14 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-8.14 8.14
# Navigate to the new working tree
cd .worktrees/backport-8.14
# Create a new branch
git switch --create backport-870-to-8.14
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 43328c81569c20502bbabbf33ce2b11fad78a842
# Push it to GitHub
git push --set-upstream origin backport-870-to-8.14
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.14

Then, create a pull request where the base branch is 8.14 and the compare/head branch is backport-870-to-8.14.