rebar.config: fix invalid branch for rebar3_diameter_compiler.git
The upstream has ranamed 'master' branch to 'main', so the old name is no longer valid. This resulted in osmo_dia2gsup build failures.
Let's use the latest stable release (0.8.0) instead. It's quite dated (2022), yet does the job. I could not find any significant changes since 0.8.0 (mostly CI and doc improvements).
