Thank you for using Bitnami and for providing feedback about the welcome message, it is using the first launch information, so this script is not showing the updated version.
php -v you are using the system package (the one included in the OS), to check the PHP version used by the WordPress stack you should run
/opt/bitnami/php/bin/php -v that is the one included by Bitnami as part of the WordPress stack.
In case of upgrading a base component (Apache, PHP, ...), and as the Bitnami stacks are self-contained and we build all the components from scratch, you will need to launch a new instance that includes those new versions and migrate the data from the old instance to this new one. We also have a guide in our documentation that explains how to do that https://docs.bitnami.com/aws/apps/wordpress/administration/backup-restore-jetpack/
At this time, we are packaging WordPress with PHP 7.2.X, so the above process will move your stack to the last PHP 7.2.X version, not 7.3.X. I just created an internal task in order to evaluate the migration to the latest branch.
Let us know if you have any questions
Carlos R. Hernández