scripts: release: list_backports: use older python dict merge method

In Python versions >= 3.9, dicts can be merged with the `|` operator.

This is not the case for python versions < 3.9, and the simplest way
is to use `dict_c = {**dict_a, **dict_b}`.

Signed-off-by: Christopher Friedt <cfriedt@fb.com>
(cherry picked from commit 3783cf83)