Context: @godofredoc pointed out that the fields in the PR template could be simplified and redesigned. Synced with @itsjustkevin and Kevin come up with [changes in this doc](https://docs.google.com/document/d/1Slppqq32toSwwxUyYTX1t6dGUQpyi0xEWxw4niGWspc/edit?resourcekey=0-v6U_9XVsMFSx0Fvx1rANhg&tab=t.0) which would improve the cherry pick template.

Github only supports a limited number of formatting options. This PR uses some workarounds and makes the following changes:
1. use divider to replace text area since Github markdown doesn't support text area
2. remove \<pre\> tags so texts can wrap around
3. use headings over bold
4. use task lists to identify the starting points of user answers

An example cherry pick template would [look like this](https://github.com/flutter/flutter/pull/143785) after the change.