Decommissioning armv7

This was decided in BoF at GUADEC 2019. See https://wiki.gnome.org/GUADEC/2019/Hackingdays/FreedesktopSdk/Notes

We will still support armv7 until 2021 with 19.08 branch.

Rationale for the removal

• Servers to maintain the armv7 builds are not free
• Usage of armv7 build is very low according to flathub's download statistic.
• Most desktop arm distributions have moved to aarch64. The exception being raspbian, but they do have betas for aarch64. So 32 bits is effectively phasing out.
• There is no big history of binaries distributed like we had with intel, and armv7 compatibility is not available on all aarch64.

Implementation

• Remove publishing
• Remove builds