Validate that the tag doesn't exist when creating a tag via the web (#33241)

Found while investigating #33210.

This line no longer makes sense because the form field "TagName" is
required, so this would mean that this code path would never be covered.
Because it isn't covered, we end up going down the "update release"
logic where we eventually set `Release.IsTag` to false (meaning it will
now be treated as a release instead of a tag).

This snapshot rewrites the condition to ensure that we aren't trying to
create a tag that already exists.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/forms/repo_form.go

@@ -651,8 +651,8 @@ type NewReleaseForm struct {
 	Target     string `form:"tag_target" binding:"Required;MaxSize(255)"`
 	Title      string `binding:"MaxSize(255)"`
 	Content    string
-	Draft      string
-	TagOnly    string
+	Draft      bool
+	TagOnly    bool
 	Prerelease bool
 	AddTagMsg  bool
 	Files      []string