Move migrate APIs to indices namespace

[pquentin]

As done in elastic/elasticsearch#120580

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
indices.cancel_migrate_reindex	⚪	Missing test	Missing test
indices.create_from	🟠	Missing recording	Missing recording
indices.get_migrate_reindex_status	⚪	Missing test	Missing test
indices.migrate_reindex	🟠	Missing recording	Missing recording

You can validate these APIs yourself by using the make validate target.

[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-3579-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c15f4cf66e0ddb5120a95fadfeaca04e7644ecbf
# Push it to GitHub
git push --set-upstream origin backport-3579-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-3579-to-8.x.

[pquentin]

💚 All backports created successfully

Status	Branch	Result
✅	8.x

Questions ?

Please refer to the Backport tool documentation

[pquentin]

💚 All backports created successfully

Status	Branch	Result
✅	8.x

Questions ?

Please refer to the Backport tool documentation