Add workaround to disable GitHub import for 15.3.1 release

Merged Stan Hu requested to merge sh-add-workaround-15.3.1 into master

Why is this change being made?

For administrators who cannot update to a security release, the workaround is a more convenient option. This came up in https://about.gitlab.com/releases/2022/08/22/critical-security-release-gitlab-15-3-1-released/#comment-5957454461.

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Edited by Stan Hu