[OSGeo] #3028: Migrate mailing lists to Mailman3

[OSGeo]

#3028: Migrate mailing lists to Mailman3
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 Reporter:  strk              |      Owner:  sac@…
     Type:  task              |     Status:  new
 Priority:  normal            |  Milestone:  Unplanned
Component:  SysAdmin/Mailman  |   Keywords:
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 See https://docs.mailman3.org/en/latest/upgrade-guide.html

 Current debian stable (12) has Mailman3 by default
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Ticket URL: <https://trac.osgeo.org/osgeo/ticket/3028>
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