Support setting the default attribute of the issue template dropdown field (#31045) (#31059)

Backport #31045 by @Zettat123

Fix #31044

According to [GitHub issue template
documentation](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-githubs-form-schema#attributes-for-dropdown),
the `default` attribute can be used to specify the preselected option
for a dropdown field.

Co-authored-by: Zettat123 <zettat123@gmail.com>

templates/repo/issue/fields/dropdown.tmpl

@@ -2,7 +2,7 @@
 	{{template "repo/issue/fields/header" .}}
 	{{/* FIXME: required validation */}}
 	<div class="ui fluid selection dropdown {{if .item.Attributes.multiple}}multiple clearable{{end}}">
-		<input type="hidden" name="form-field-{{.item.ID}}" value="0">
+		<input type="hidden" name="form-field-{{.item.ID}}" value="{{.item.Attributes.default}}">
 		{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 		{{if not .item.Validations.required}}
 		{{svg "octicon-x" 14 "remove icon"}}