MAINTAINERS: Adjust migration documentation files

MAINTAINERS: Adjust migration documentation files

Commit 8cb2f8b172e7 ("docs/migration: Create migration/ directory")
changed migration documentation file structure but forgot to update the
entries in the MAINTAINERS file.

Commit 4c6f8a79ae53 ("docs/migration: Split 'dirty limit'") extracted
dirty limit documentation to a new file without updating dirty limit
section in MAINTAINERS file.

Fix the above.

Fixes: 8cb2f8b172e7 ("docs/migration: Create migration/ directory")
Fixes: 4c6f8a79ae53 ("docs/migration: Split 'dirty limit'")
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Link: https://lore.kernel.org/r/20240407081125.13951-1-avihaih@nvidia.com
Signed-off-by: Peter Xu <peterx@redhat.com>