[flutter_local_notifications] Update gson package for vulnerabilities

[briannaAndCo]

Fixes CVE-2022-25647.

This updates an android jar to a version without the vulnerability. Vulnerability is considered HIGH and more info is here

[MaikuB]

Thanks a lot for the PR. I noticed there's 2.9.0 that was out in February so curious if you know there's a why 2.9.0 shouldn't be used instead. Not following the development of gson but as you knew about this issue i thought I'd ask in case you might know

[briannaAndCo]

I picked 1.8.9 because it's the first version without the vulnerability. And while I know jumping minor versions shouldn't cause issues, it sometimes can in my experience, so it seemed safer to stay with the same minor version if possible.

[MaikuB]

Thanks that was my guess but wanted to check