Revert the libjpeg-turbo update and roll back to 2.1.5.1

[mattleibow]

Description of Change

This PR reverts the update to libjpeg-turbo external dependency because of several encoding issues.

Reverts #2583

Issues Fixed

• Fixes [BUG] Xamarin : Broken Bitmap.Encode on real devices #2643

API Changes

None.

Behavioral Changes

None.

Required skia PR

mono/skia#115

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard
• Updated documentation

[github-actions]

The backport to release/2.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-release/2.x release/2.x
# Navigate to the new working tree
cd .worktrees/backport-release/2.x
# Create a new branch
git switch --create backport/pr-2699-to-release/2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6a51a6bca0770e4fd0d16202243fee8b922cd9e8
# Push it to GitHub
git push --set-upstream origin backport/pr-2699-to-release/2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/2.x

Then, create a pull request where the base branch is release/2.x and the compare/head branch is backport/pr-2699-to-release/2.x.