Support setting the `default` attribute of the issue template dropdown field (#31045...

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>