[DATALAD RUNCMD] Replace youtube URL with the one with @dandiarchive …

…since looks better/easier to remember

=== Do not change lines below ===
{
 "chain": [],
 "cmd": "git-sedi channel/UCsLLBNhtcV-wL8cCZWveDCA @dandiarchive",
 "exit": 0,
 "extra_inputs": [],
 "inputs": [],
 "outputs": [],
 "pwd": "."
}
^^^ Do not change lines above ^^^

dandiapi/api/templates/api/mail/approved_user_message.txt

@@ -8,7 +8,7 @@ Please use the following links to post any questions or issues.
 
 Discussions: https://github.com/dandi/helpdesk/discussions
 Issues: https://github.com/dandi/helpdesk/issues/new/choose
-YouTube: https://www.youtube.com/channel/UCsLLBNhtcV-wL8cCZWveDCA
+YouTube: https://www.youtube.com/@dandiarchive
 (please Subscribe)
 
 Thank you for choosing DANDI for your neurophysiology data needs.

dandiapi/api/templates/api/mail/registered_message.txt

@@ -16,7 +16,7 @@ Please use the following links to post any questions or issues.
 
 Discussions: https://github.com/dandi/helpdesk/discussions
 Issues: https://github.com/dandi/helpdesk/issues/new/choose
-YouTube: https://www.youtube.com/channel/UCsLLBNhtcV-wL8cCZWveDCA
+YouTube: https://www.youtube.com/@dandiarchive
 (please Subscribe)
 
 Thank you for choosing DANDI for your neurophysiology data needs.