Replace issue.go dependency on archived project github.com/fatih/structs

## Is your feature request related to a problem? Please describe.

issue.go currently imports the `github.com/fatih/structs` module which is an [archived project ](https://github.com/fatih/structs) no longer receiving maintenance updates. The last update to the project was over 3 years ago. This is causing dependency scanning software I am using (Sonatype IQ) to flag go-jira as a potential security risk.

## Describe the solution you'd like

Replace the dependency on `github.com/fatih/structs` using the standard library. I only see this dependency being used once [here](https://github.com/andygrunwald/go-jira/blob/e8880eb25076e18451dd39c0060b2b0ae8bcfa89/issue.go#L152).

## Describe alternatives you've considered

Find an active project to use in place of `github.com/fatih/structs` that provides the same functionality

## Additional context