MFS11 320824: Add deprecation notices for gdb and kgdb.

Even though gdb and kgdb may not be removed for 12.0 on some architectures,
the notice is unconditional as these tools will likely be removed at some
point in the future when adequate replacements are available (gdb in ports
or lldb in base).

Approved by: re (gjb)
Relnotes: yes