[Refactor]: updated dependency for focalboard server

[Rajat-Dabade]

Fixes: https://mattermost.atlassian.net/browse/MM-58301

This PR Includes:

Dependency Update:

• Updated the Mattermost server dependency from github.com/mattermost/mattermost-server/v6/public to github.com/mattermost/mattermost/server/public.

Go Version Upgrade:

• Bumped the Go version from 1.19 to 1.21.

GitHub Workflow Updates:

• Updated Go dependencies in GitHub workflows.

Code Cleanup:

• Removed unnecessary code files and their related implementations across Focalboard:
  • cloud.go: cloud limits are no longer applied as part of plugins, also removed the server-side code which was using cloud limits.
  • limit.go: Removed for the same reason as cloud.go.
  • insight.go: Removed as insights on boards are no longer supported.
  • statistics.go: Removed as statistics routes are no longer supported.

[marianunez]

Great work @Rajat-Dabade! Let some comments below and there is still more limits and insights code we can delete.

[marianunez]

Looking great @Rajat-Dabade

Left some minor comments but one thing we do want to double check is that some settings are coming through the clientConfig.FeatureFlags which we do want to keep. The only FeatureFlag we don't want to check any longer is the one where it determined if Boards was running as a product in the server or not (the old MPA model)

[marianunez]

Aside the minor revert of the boardsEditor setting, everything LGTM. Great work @Rajat-Dabade 🎉

[Rajat-Dabade]

Reverted the changes for BoardEditor setting back. Thanks 🙌 @marianunez