Bump avro once their CMake is updated

[SwooshyCueb]

We currently provide Avro 1.11, which was written to be built with CMake 3.5. Compatibility with CMake 3.5 has been ripped out of newer versions of CMake, so let's update. There are no equivalent distro-provided packages, so updating this should be fine.

[SwooshyCueb]

It looks like I misread the commit history of the upstream avro repo. The CMake updates we need are not yet in a tagged release.
Let's patch for now and wait for 1.12 to mature before updating. If the changes we need never land in 1.12.1+, let's instead wait for a mature 1.13 release.