Support setting the `default` attribute of the issue template dropdown field (#31045)
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.
(cherry picked from commit 7ab0988af140aa3e0204979765f75961f1dc9c11)
authored by