Add webkit browser back for playwright tests

[LinHuiqing]

Webkit browser was removed from browsers tested by playwright tests as the encrypt-submission test suite was failing for webkit (refer to the Playwright Test run here: https://github.com/opengovsg/FormSG/actions/runs/4954262530/jobs/8863701326). All tests pass on local for webkit though, so I'm not sure what is wrong or how to debug this?

Basically, when the encrypt-submission test suite is run on GitHub actions, the download event for the secret key never happens so the test times out.

Note: this is part of a series of issues created due to the PR to try to fix Playwright tests (#6208).