Update "Installation from source" guide for 8.15.0
Following the steps from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Following the steps from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1.