Follow-up for reference pattern fix

Described in !65847 (comment 642304813)

Concern

We have fixed the reference_pattern for milestone detection in !65847 (diffs). However, we rely on a similar regex for iterations.

It is a not a problem now because IterationReferenceFilter does not trigger ReferenceCache to load parents by using an incorrect pattern.

Proposal

1. Consider adding a reference cache to iterations and fix the problem - (Will be done in #327630 (closed))

2. If not possible, fix the regex and verify that it works as expected.

Solution

Fix regex as it was done for milestones.