[MM-232] Update plugin with respect to phase 1 upgrades

[ayusht2810]

Summary

Updated plugin with respect to phase 1 upgrades

• Updated go version from 1.18 to 1.21
• Updated build folder to sync with starter template
• Updated package in go modules to use https://pkg.go.dev/github.com/mattermost/mattermost/server/public
• Updated Makefile
• Updated version in nvmrc file and update package-lock.json file.
• Updated developer docs link in the readme file

Ticket Link

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 22.52%. Comparing base (bbf7349) to head (d2b4575).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #111 +/- ## ========================================== - Coverage 23.12% 22.52% -0.60% ========================================== Files 6 6 Lines 506 506 ========================================== - Hits 117 114 -3 - Misses 372 374 +2 - Partials 17 18 +1 ```

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