[MM-53944] Make use of github.com/mattermost/mattermost/server/public

[hanzei]

Summary

This PR makes use of the newly created go module in https://pkg.go.dev/github.com/mattermost/mattermost/server/public.

See mattermost/mattermost-plugin-demo#163 for a similar migration.

Ticket Link

https://mattermost.atlassian.net/browse/MM-53944

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (8638c32) 32.93% compared to head (9db3b61) 32.94%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #393   +/-   ##
=======================================
  Coverage   32.93%   32.94%           
=======================================
  Files          22       22           
  Lines        4014     4013    -1     
=======================================
  Hits         1322     1322           
+ Misses       2561     2560    -1     
  Partials      131      131           

Files Changed	Coverage Δ
server/api.go	21.17% <ø> (ø)
server/cluster.go	0.00% <ø> (ø)
server/command.go	24.27% <ø> (ø)
server/configuration.go	33.33% <ø> (ø)
server/flow.go	0.00% <ø> (ø)
server/main.go	0.00% <ø> (ø)
server/plugin.go	14.48% <ø> (ø)
server/telemetry.go	0.00% <ø> (ø)
server/webhook.go	40.30% <ø> (ø)

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[mickmister]

LGTM

[hanzei]

@spirosoik Would you mind giving this PR a review so we can get it into 1.7.0?

[spirosoik]

@hanzei overall looks good to me but I don't know what could be the impact of this change. may we need to include one more pair of eyes here. what do you think?

[hanzei]

I did a smoke test of the change (compile the plugin, start it, and run a slash command). Is that enough? More complex issues might also be caught during release testing.