gdb/NEWS: Move entries about MI v3 and multi-loc breakpoints after GDB 8.3

gdb/NEWS: Move entries about MI v3 and multi-loc breakpoints after GDB 8.3

This is a followup on the following commit...

    commit b4be1b0648608a2578bbed39841c8ee411773edd
    Date:   Wed Mar 13 15:13:03 2019 -0400
    Subject: Fix MI output for multi-location breakpoints

... which mistakenly added NEWS entries in the "in gdb-8.3" section,
rather than in the "since gdb-8.3" one.

gdb/ChangeLog:

	* NEWS: Move entries about default MI version now being
        version 3, and about the GDB/MI fix for multi-location
        breakpoints to the "since GDB 8.3" section.