Update app.fyreplace.client

When adding my app to the repository, I was using the latest and greatest Gradle version. Since new versions of Gradle have first to be added to this repository to be used, the version was patched on the fly to use the last Gradle 5.x release instead.

I just switched to using the version recommended by Android Studio when updating the Gradle plugin (6.1.1 right now), so the Gradle version doesn't need to be pinned anymore; it should always be a version that has already been merged and that is available for F-Droid builds.

EDIT: The change I'm proposing right now would change the way older versions of the app have been built, so technically it breaks their reproducibility; although it's only affecting the build system and not the source code. I don't know if that's a problem, and if it is, then I would like to know what is the best way to go about removing the init line from the builds