Update release process to include diffs of deployment repos

In the update docs, we can start including a link to a diff that shows all changes since the last release: for example, https://sourcegraph.com/github.com/sourcegraph/deploy-sourcegraph-docker/-/compare/3.40...3.41. This should be included for docker-compose and kustomize deployments.

These diffs will include some irrelevant changes, but should provide customers with an easy way to scan the changes and see if anything significant has to be incorporated in their own deployment.

Note: Sourcegraph only supports diffs using . . ., which compares two changes starting at a common ancestor. The diff shown will be slightly misleading for the deploy repos since we never merge release branches back into mainline.

       git diff [<options>] <commit>..<commit> [--] [<path>...]
           This is synonymous to the earlier form (without the ..) for viewing the changes between two arbitrary <commit>. If <commit>
           on one side is omitted, it will have the same effect as using HEAD instead.

       git diff [<options>] <commit>...<commit> [--] [<path>...]
           This form is to view the changes on the branch containing and up to the second <commit>, starting at a common ancestor of
           both <commit>.  git diff A...B is equivalent to git diff $(git merge-base A B) B. You can omit any one of <commit>, which
           has the same effect as using HEAD instead.

sourcegraph / sourcegraph-public-snapshot

Update release process to include diffs of deployment repos #34849