Add Gitaly HA beta to 12.10 release post

Why is this change being made?

This item got missed in the 12.10 release post

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

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