RP item: cherry-pick and fix Gitaly post

Why is this change being made?

This MR cherry-picks the two commits (a8a25f62408 for creation and 0915dd3e1f8 for revisions) to grab this file, which was placed in the right folder (13_1), but on the wrong branch (master):

data/release_posts/13_1/gitaly-cluster-read-distribution.yml

and copy it into the folder on the release-13-1 branch so it will be visible in the review app. I've also layered on a couple of minor editorial revisions - one to the availability line (while probably syntactically correct it wasn't formatted like the other lines) and some grammar changes to get us to active voice.

cc @jramsay as this was your RP item

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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