Fix Incomplete Breaking Changes RSS Feed

Why is this change being made?

The Breaking Changes RSS Feed is skipping breaking changes whose associated merge requests are missing labels used by the feed generator. This MR addresses the issue by pulling breaking changes directly from the gitlab-org/gitlab repository rather than using a merge request search.

This MR also changes the breaking change feed entry links to the originating issue instead of the breaking change's merge request, which is being done for technical reasons related to changing the lookup method, but should have an added benefit as the breaking change issues have more context behind the change compared to their associated MRs.

Author Checklist

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