Notify of outstanding forward ports on the forward port instead of the source

[xmo-odoo]

Req'd by xdo.

When the reminder was overhauled with #198 decision was made to only warn on the original PR because

there's no need to explicitly at-tag the author or reviewer, they're likely following the original PR still

however,

• later on at-tagging the author and reviewer were added anyway
• this affects other followers of the original PR, not just the author and reviewer

Furthermore this increases the number of steps necessary to fix the issue, as the user trying to handle it has to go to the PR, check the reminder comment, open the linked PRs, then handle them.

It might be a better idea to ping the outstanding forward ports, this removes a step from their handling, and avoid pinging all the followers of the original task, most of whom probably don't care anyway.