Update flathub? Base SDK out of date

[kmoad]

When updating from flathub, I get this message

Info: org.freedesktop.Platform//18.08 is end-of-life, with reason:
   The Freedesktop SDK 18.08 runtime is no longer supported as of 31th August 2020. 
   Please ask your application developer to migrate to a supported platform.
Applications using this runtime:
   com.github.artemanufrij.regextester

Could you update the app on flathub to use a supported SDK? Any hopefully also update the app as noticed by #22 ?

[kmoad]

Issue is being worked on #27