Fixes broken security:prepare task

Mayra Cabrerarequested to merge
fixes-broken-security-task into master

What does this MR do?

From https://gitlab.com/gitlab-org/release-tools/-/jobs/430098423, create_issue method on GitLabClient still assumes there's only one version per issue and retrieves the milestone from that version.

This is no longer true for security releases, in which all tasks are concentrated in a single issue (https://gitlab.com/gitlab-org/release-tools/issues/380)

This MR adds milestone_name method on Issuable, Issue and PatchIssue to fix the problem

Edited by Mayra Cabrera

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