Make Draft working as a WIP prefix

The issue was that only the actual title was converted to uppercase, but
not the prefix as specified in `WORK_IN_PROGRESS_PREFIXES`. As a result,
the following did not work:

    WORK_IN_PROGRESS_PREFIXES=Draft:,[Draft],WIP:,[WIP]

One possible workaround was:

    WORK_IN_PROGRESS_PREFIXES=DRAFT:,[DRAFT],WIP:,[WIP]

Then indeed one could use `Draft` (as well as `DRAFT`) in the title.
However, the link `Start the title with DRAFT: to prevent the pull request
from being merged accidentally.` showed the suggestion in uppercase; so
it is not possible to show it as `Draft`. This PR fixes it, and allows
to use `Draft` in `WORK_IN_PROGRESS_PREFIXES`.

Fixes go-gitea#19779.

models/pull.go

@@ -588,7 +588,7 @@ func (pr *PullRequest) IsWorkInProgress() bool {
 // HasWorkInProgressPrefix determines if the given PR title has a Work In Progress prefix
 func HasWorkInProgressPrefix(title string) bool {
 	for _, prefix := range setting.Repository.PullRequest.WorkInProgressPrefixes {
-		if strings.HasPrefix(strings.ToUpper(title), prefix) {
+		if strings.HasPrefix(strings.ToUpper(title), strings.ToUpper(prefix)) {
 			return true
 		}
 	}