* Refactor: Use generic to change func StringInSlice to Contains
* Refactor: Use generic to change func stringNotInSlice to Contains
Make utils.go (dir server/public/utils) and func Contains
* Refactor: Move func Contains from channels/utils to public/utils
Move func Contains from channels/utils to public/utils
Fix import declarations line
* Docs: Add a description of the Contains function
* Test: add TestContains
Add a test code for a Contain function
It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.
https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.
For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```
