Enhance the result of PlanPreview Action to distinguish multiple projects

[t-kikuc]

What would you like to be added:

1. Distinguish results by their Project clearly
2. Avoid minimizing the result of other Projects

Why is this needed:

When a user requests PlanPreview from one repository to multiple PipeCD Projects,

1. users cannot tell which Project the result comes from. That is inconvenient.
2. the earlier result will be minimized due to this logic of <!-- pipecd-plan-preview-->:
   

   pipecd/tool/actions-plan-preview/github.go

   Lines 199 to 211 in b478c61

   	// Expect comments to be sorted in ascending order by created_at
   	func filterLatestPlanPreviewComment(comments []issueCommentQuery) *issueCommentQuery {
   	const planPreviewCommentStart = "<!-- pipecd-plan-preview-->"
   	for i := range comments {
   	comment := comments[len(comments)-i-1]
   	if strings.HasPrefix(string(comment.Body), planPreviewCommentStart) {
   	return &comment
   	}
   	}
   	return nil
   	}

[t-kikuc]

The same as this one:
#5898 (comment)