Read migration version from directory of files

As part of the implementation of this issue: gitlab-org/gitlab#212425 (closed) in MR: gitlab-org/gitlab!30109 (merged) the way schema migrations are tracked will change. This updates the script to pull the information from the new tracking method if the directory exists.

Make the wait-for-deps script capable of reading the latest schema version number from a list of files in a db/schema_migrations directory. If the directory is not present, it will continue to read the changes according to the existing logic.