Change the ABI to glibmm-2.60
So we can use the 2.58 version numbers for stable releases in the glibmm-2.4 ABI series. We don't need to release a stable ABI-parallel glibmm until we need to release gtkmm 4.0, and that won't happen until GTK+ 4.0.0 happens, and we don't know when that might be. We've done similar ABI name changes several times before.
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