Bump elastictransport to 8.6.1

[Anaethelion]

As titled.

[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-954-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 4dd413532dc4346b82f770f466f633b8e102a12b
# Push it to GitHub
git push --set-upstream origin backport-954-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-954-to-8.x.

[github-actions]

The backport to 8.18 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-8.18 8.18
# Navigate to the new working tree
cd .worktrees/backport-8.18
# Create a new branch
git switch --create backport-954-to-8.18
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 4dd413532dc4346b82f770f466f633b8e102a12b
# Push it to GitHub
git push --set-upstream origin backport-954-to-8.18
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.18

Then, create a pull request where the base branch is 8.18 and the compare/head branch is backport-954-to-8.18.