[MM-38146] Use mattermost-server/v6

[hanzei]

Summary

This PR updates the dependency on github.com/mattermost/mattermost-server from v5 to v6. This ensures the v6 breaking changes propagate quickly to developers using this plugin.

Please note that the plugin still works against v5 servers.

Ticket Link

https://mattermost.atlassian.net/browse/MM-38146

[codecov-commenter]

Codecov Report

Merging #169 (71aebaa) into master (7f5d45a) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
- Coverage   41.57%   41.49%   -0.09%     
==========================================
  Files           6        6              
  Lines         671      670       -1     
==========================================
- Hits          279      278       -1     
  Misses        375      375              
  Partials       17       17              

Impacted Files	Coverage Δ
server/autolinkplugin/command.go	0.00% <0.00%> (ø)
server/autolinkplugin/config.go	37.58% <ø> (ø)
server/autolinkplugin/plugin.go	91.93% <ø> (-0.07%)	⬇️

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[hanzei]

@streamer45 @mickmister This is ready for review

[streamer45]

Looks good 👍

[hanzei]

@DHaussermann This PR is ready for a round of smoke testing. Maybe we can even defer that to the release testing.

[hanzei]

@DHaussermann I'm going to defer the QA approval to release testing.