I support only the last published version and the last development version (last commit).
To report a vulnerability, add a new draft security advisory at https://github.com/vrana/adminer/security/advisories/new.
I handle security issues with top priority. If you don't hear from me in a week then please ping the bug. Once I accept the bug, the fix should be available and new version released within days. I will mark the bug as public after releasing a new version or declining the bug.