[db] Track db versions in central files

[MariemBaccari]

• Fixes Keep track of the db version in a central file #351 by using text files under /build/db_schemas/version to keep track of the database version.
• The files indicated in the migration documentation will retrieve the version from there.
• The specific case of the helm-charts update requires copying the text files in that directory at each version update as Helm does not support reading from files outside of its directory. This is done using a simple script that should be run after the text files are updated.
• The readme instructions are also updated.