gdb: move remaining ChangeLogs to legacy files

In commit:

  commit f069ea4
  Date:   Sat Jul 3 16:29:08 2021 -0700

      Rename gdb/ChangeLog to gdb/ChangeLog-2021

The gdb/ChangeLog file was renamed, but all of the other ChangeLog
files relating to gdb were left in place.

As I understand things, the no ChangeLogs policy applies to all the
GDB related directories, so this commit renames all of the remaining
GDB related ChangeLog files.

As with the original commit, the intention behind this commit is to
hopefully stop people merging ChangeLog entries by mistake.

The renames carried out in this commit are:

    gdb/doc/ChangeLog -> gdb/doc/ChangeLog-1991-2021
    gdb/stubs/ChangeLog -> gdb/stubs/ChangeLog-2012-2020
    gdb/testsuite/ChangeLog -> gdb/testsuite/ChangeLog-2014-2021
    gdbserver/ChangeLog -> gdbserver/ChangeLog-2002-2021
    gdbsupport/ChangeLog -> gdbsupport/ChangeLog-2020-2021

Author: T-J-Teru