Switch to newer grondag repo in build.gradle

[colbiedison]

I was unable to build the mod from source, due to an internal server error from https://grondag-repo.appspot.com/. Grondag said he doesn't know what's going on with that old repo, and told me to use the newer one at https://maven.dblsaiko.net/. I changed it in the build script and it worked.

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