gdb/testsuite: fix license on recently added file

gdb/testsuite: fix license on recently added file

The license header on a file I recently contributed was incorrect.
The file was added in commit:

  commit 087969169836f802a09b1cd0502d2f22d7a8f7dc
  Date:   Tue May 23 11:25:21 2023 +0100

      gdb: handle core files with .reg/0 section names

The problems were:

  - GPLv2 instead of GPLv3,
  - Use the FSF postal address rather than their URL.

Nobody else has touched the file since I merged it, so I don't believe
there are any problems with me changing the license, this commit does
just that.