gdb/NEWS: reorder some entries in the NEWS file

I spotted two entries in the NEWS file that I believe are in the wrong
place, these are:

  - An entry about MI v1 being deprecated, this feels like it should
    be the first entry under the 'MI changes' heading, and

  - An entry for the $_shell convenience function which is currently
    under the 'New commands' heading (sort of), when I think this
    should be listed in the general news section.