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

[ayusht2810]

Summary

Updated plugin with respect to phase 1 upgrades

• Updated go version from 1.13 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 developer docs link in the readme file

Ticket Link

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 6.32%. Comparing base (3e3dae9) to head (8595a3c).

Files	Patch %	Lines
server/telemetry.go	0.00%	11 Missing ⚠️
server/plugin.go	0.00%	4 Missing ⚠️
server/configuration.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #244      +/-   ##
=========================================
- Coverage    6.42%   6.32%   -0.11%     
=========================================
  Files          11      10       -1     
  Lines        1712    1708       -4     
=========================================
- Hits          110     108       -2     
+ Misses       1594    1592       -2     
  Partials        8       8              

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[mickmister]

@ayusht2810 Can you change the Go version in the Playwright test CI to 1.21 here? That test is failing currently because of the version mismatch

https://github.com/mattermost/mattermost-plugin-todo/blob/3e3dae93c6d9d7225a9d4da981b09b560577e16b/.github/workflows/playwright.yml#L13

[Kshitij-Katiyar]

@ayusht2810 Can you change the Go version in the Playwright test CI to 1.21 here? That test is failing currently because of the version mismatch

https://github.com/mattermost/mattermost-plugin-todo/blob/3e3dae93c6d9d7225a9d4da981b09b560577e16b/.github/workflows/playwright.yml#L13

@mickmister Done and the Playwright test is working fine.