You might need to engage with your local Moodle Partner for this - Moodle Partners are often asked to provide security documentation for our hosted solutions too - it can also depend on the jurisdiction you are in which is why engaging with your local Moodle partner can help as they may have needed to satisfy similar requirements for other government institutions within your country.
Various ISO policies like ISO 27001 and ISO 9001 can be important, but also regular pen-testing reports - but these depend more specifically on how you have implemented Moodle and the surrounding processes and systems rather than Moodle itself.
Moodle publishes some information here that may provide some useful information as well,.
https://moodle.com/security-privacy/
however if you are looking for a "certification" for your self-hosted solution - you will likely need to engage someone commercially (pay them) to audit your system and provide this for you - and who that can be may depend on your government's requirements.
Various ISO policies like ISO 27001 and ISO 9001 can be important, but also regular pen-testing reports - but these depend more specifically on how you have implemented Moodle and the surrounding processes and systems rather than Moodle itself.
Moodle publishes some information here that may provide some useful information as well,.
https://moodle.com/security-privacy/
however if you are looking for a "certification" for your self-hosted solution - you will likely need to engage someone commercially (pay them) to audit your system and provide this for you - and who that can be may depend on your government's requirements.