Hi Alex Marshall, thanks for raising your issue. Teams connectors are now deprecated, you're supposed to use the Workflows app as a replacement.
Details are here: https://devblogs.microsoft.com/microsoft365dev/retirement-of-office-365-connectors-within-microsoft-teams/
You can follow below document to create incoming webhooks with Workflows for Microsoft Teams.
https://support.microsoft.com/en-us/office/create-incoming-webhooks-with-workflows-for-microsoft-teams-8ae491c7-0394-4861-ba59-055e33f75498
Thanks,
Prasad Das
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