gdb: Update SECURITY.txt to mention extension scripts and internal errors

gdb: Update SECURITY.txt to mention extension scripts and internal errors

Given the recent CVE filed for GDB (CVE-2024-36699), I decided to update
the gdb/SECURITY.txt to be more explicit about some details. Specifically,
we now explicitly say that internal errors aren't security
vulnerabilities, and mention that users should review plugins before
running them, and under which conditions a plugin can cause a security
bug.

Reviewed-By: Tom Tromey <tom@tromey.com>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>