[bug] Jira Server user search API triggers 404 error with accountID

## Describe the bug

Current Jira Server API has user search method which unavailable after
https://github.com/andygrunwald/go-jira/commit/f50cb07b297d79138b13e5ab49ea33965d32f5c1

## To Reproduce

1. Use Jira Server API
2. Try to search user with accoundID `user, _, error := client.User.Get(accountID)`
3. Catch error `Either the 'username' or the 'key' query parameters need to be provided: request failed. Please analyze the request body for more details. Status code: 404`

## Expected behavior

No reproduce 404 error with valid request

https://docs.atlassian.com/software/jira/docs/api/REST/8.9.0/#api/2/user-findUsers
## Possible Solution

Add both (Cloud and Server) compatibility

## Your Environment

Include as many relevant details about the environment you experienced the problem in

* go-jira version (git tag or sha): https://github.com/andygrunwald/go-jira/commit/de60bbed3f0c62e5933c8927cddf1d4a7ee20f75
* Go version (`go version`): v1.14.4
* Jira version: v8.5.3
* Jira type (cloud or on-premise): Server
* Operating System and version: macOS 10.15.4