Change handling of over-long DAP variables requests

In PR dap/33228, we changed gdb to gracefully report an error if a DAP
'variables' request asked for more variables than had been reported.

This behavior was clarified in the spec, see

    https://github.com/microsoft/debug-adapter-protocol/issues/571

This patch changes gdb to conform to the specified approach, namely
truncating the list rather than erroring.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33228
Approved-By: Andrew Burgess <aburgess@redhat.com>