Use new unsubscribe link

What does this MR do?

The unsubscribe email we generate uses a +, such as incoming-1234567890abcdef1234567890abcdef+unsubscribe@incoming.gitlab.com. However this will not work when using a catch-all email address that doesn't support subaddressing.

We now use a -, such as incoming-1234567890abcdef1234567890abcdef-unsubscribe@incoming.gitlab.com

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/29951

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• Security reports checked/validated by reviewer