Fix links to the migrating guide.

[mikelittle]

All the upgrading guides had a broken link.

[hm-backport]

The backport to v12-branch failed:

Commits ["6d306aee6abd888bc47d54399fc8787ccd158337"] could not be cherry-picked on top of v12-branch

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport v12-branch
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 6d306aee6abd888bc47d54399fc8787ccd158337
# Create a new branch with these backported commits.
git checkout -b backport-520-to-v12-branch
# Push it to GitHub.
git push --set-upstream origin backport-520-to-v12-branch
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is v12-branch and the compare/head branch is backport-520-to-v12-branch.

[hm-backport]

The backport to v13-branch failed:

Commits ["6d306aee6abd888bc47d54399fc8787ccd158337"] could not be cherry-picked on top of v13-branch

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport v13-branch
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 6d306aee6abd888bc47d54399fc8787ccd158337
# Create a new branch with these backported commits.
git checkout -b backport-520-to-v13-branch
# Push it to GitHub.
git push --set-upstream origin backport-520-to-v13-branch
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is v13-branch and the compare/head branch is backport-520-to-v13-branch.

[hm-backport]

The backport to v14-branch failed:

Commits ["6d306aee6abd888bc47d54399fc8787ccd158337"] could not be cherry-picked on top of v14-branch

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport v14-branch
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 6d306aee6abd888bc47d54399fc8787ccd158337
# Create a new branch with these backported commits.
git checkout -b backport-520-to-v14-branch
# Push it to GitHub.
git push --set-upstream origin backport-520-to-v14-branch
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is v14-branch and the compare/head branch is backport-520-to-v14-branch.