Update System.Text.Json to remove a critical vulnerability

[LauraArmitage]

Apologies if I've done this incorrectly, I've not reported an issue on an open source project before

Also the security pages have not been updated past the default to tell me how to report a vulnerability.

Describe the bug
When including RestSharp in my project the default dependencies include System.Text.Json at v5.0.0 which includes System.Text.Encodings.Web at v5.0.0

System.Text.Encodings.Web v5.0.0 has a security vulnerability as documented here: GHSA-ghhp-997w-qr28

v5.0.1 is a patched version according to that page so hopefully it's an easy, non-breaking change to update

To Reproduce
Create a new project and add the latest (108.0.1 as of reporting) version of RestSharp