Support for Heroku-18 stack

[martofeld]

I get that Grails 2.4.4 has been deprecated for a while, but I have an app written in grails with a client who doesn't want to pay for having it upgraded or migrated. Since yesterday the app started to fail when connecting to the postgress DB and seems related to this https://stackoverflow.com/questions/66623937/heroku-postgres-suddenly-appeared-error. While trying to fix it I'm now unable to deploy any updates to the app (note that the last deploy was on nov 2019 prior to the forced update to Heroku 18). As I'm not an expert in buildpacks, does anyone know what needs to be done to support Heroku 18 and be able to deploy?

Currently when trying to deploy I get

-----> Building on the Heroku-18 stack
-----> Using buildpack: https://github.com/heroku/heroku-buildpack-grails#legacy
-----> Grails app detected
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
 !     Push rejected, failed to compile Grails app.
 !     Push failed