Convert GitHub issue templates to the new YAML format

[gctucker]

There is a new way to define GitHub issue templates including some input forms. It's also possible to configure the template selector now with additional features.

This might take a bit of work to convert the current Markdown templates but it'll be a good improvement for user adoption going forward.

[nuclearcat]

Closing as no longer applicable. The Markdown templates this issue targeted—API staging access, kernel bugs, and user stories—were removed as outdated by commit 384036d. The remaining TSC-member template was added later and was not part of the original conversion scope; a new focused issue can be opened if converting that template is desired.