Updated bulk move issues milestone in the docs

What does this MR do and why?

Follow-up from !95688 (merged).

In this MR I update/fix the milestone mentioned in the docs related to the bulk move issues feature

Related to Bulk move issues to a different project (#15991)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/user/project/issues/managing_issues.md

@@ -342,7 +342,7 @@ To move an issue:
 
 #### From the issues list
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15991) in GitLab 15.5.
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15991) in GitLab 15.6.
 
 You can move multiple issues at the same time when you’re in a project.
 You can't move tasks or test cases.