prod: add team update maintenance notice

Announce maintenance and downtime for feedstock teams.

Author: beckermr

news/2026-04-21-team-update-maint.md

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+# Feedstock Team Update Maintenance & Downtime
+
+We need to do maintenance on the feedstock GitHub teams that requires some downtime.
+During this downtime, feedstock teams will not be updated. If you need help with an
+urgent, feedstock team-related maintenance item, please contact `@conda-forge/core`.