As my partner @michiel pointed, the Bitnami Moodle stack ships PHP 7.0.25, so you don't have to worry about any updates.
Also, as we compile everything for this stack (PHP, Apache, etc.) you won't be able to update PHP installing through a package manager or downloading any new version from internet. If in the future you need to upgrade any component, the recommended way to achieve that is migrating your data to a new stack. You will have a fresh instance with all the components updated to the latest version available.
You have some Bitnami docs about migrating your installation in case you require them in the future.
We proceed to close this case as we consider this is resolved. For further issues, please, do not hesitate on opening another thread.
Thank you and best regards,
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