[MM-102] Update plugin with respect to phase 1 upgrades

[ayusht2810]

Summary

Updated plugin with respect to phase 1 upgrades

• Updated Makefile
• Updated build folder
• Updated version in the nvmrc file.
• Updated developer docs link in the readme file
• Updated package in go modules to use https://pkg.go.dev/github.com/mattermost/mattermost/server/public.
• Fixed test cases

Ticket Link

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de2e85a) 23.71% compared to head (4277468) 23.71%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #351   +/-   ##
=======================================
  Coverage   23.71%   23.71%           
=======================================
  Files          62       62           
  Lines        3087     3087           
=======================================
  Hits          732      732           
  Misses       2274     2274           
  Partials       81       81           

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[mickmister]

@fmartingr Heads up that this PR will likely affect the gcal plugin's dependencies. (and yes I know I still need to address #267 (comment) 🙂)