Update community contribution link

Why is this change being made?

Please see the 15.8 retrospective note here: #14374 (comment 1276894793)

This MR updates the link for checking the number of merged community contributor MRs per release cycle. The old link used a search within GitLab that only counted MRs tagged with the milestone label but the majority of MRs do not have this label. The new link uses a Sisense chart that gives the full number of merged community contributor MRs during the release cycle.

If there are any other places this instruction needs to be updated please make note, thank you! :-)

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