ci: upgrade sauce-connect-action to v3

[GraemeWatt]

Following on from PR #533, we were using sauce-connect-action@main which broke after saucelabs/sauce-connect-action#53 was merged into the main branch, since the restUrl parameter was not being passed when starting the Docker container for Sauce Connect. The problem was fixed by instead pinning sauce-connect-action@v2. The Sauce Connect Proxy option --rest-url is deprecated in favour of --region, but specifying region: eu-central instead of restUrl: https://api.eu-central-1.saucelabs.com/rest/v1 in the ci.yml file did not work with either the main or v2 branch of sauce-connect-action. We should switch to v3 of sauce-connect-action after it is tagged and investigate if region works instead of restUrl. If not, the bug should be reported to the maintainers of sauce-connect-action. It could be checked whether the region option works when specified inside a configFile or as a SAUCE_REGION environment variable. Also, we should switch from tunnelIdentifier to tunnelName.