Consider skipping backport issue creation if original issue is not assigned a milestone

[sbrannen]

When I submitted spring-projects/spring-framework@0adabdb to spring-framework 4.3.x to close spring-projects/spring-framework#22520, I had initially forgotten to assign that PR to the next 4.3.x milestone.

Consequently, the bot automatically created spring-projects/spring-framework#22575 for the 4.3.x branch, under the assumption that I was in the process of performing a backport.

In this particular scenario, however, the merged PR and subsequent commit that closed the PR were only ever intended to be applied to the 4.3.x branch.

Thus, I was a bit confused that the bot created a backport issue for the same branch (4.3.x).

After some discussions with @rwinch, I think it would be better if the bot skipped the creation of the backport issue if the original PR/issue is not assigned a specific version (via a GitHub milestone). Instead, the bot could issue a warning in the form of a comment to the original issue indicating that the user should potentially take some manual actions.