tasks: Move to Fedora 39

[martinpitt]

I locally built this and ran the podman tests successfully. I'm sure that there will be some fallout wrt. pixel tests and such, I'll go through them.

• Rebuild and deploy
• check cockpit: test: Update pixel references to Fedora 39 tasks container, fix tests for Chromium 119 cockpit#19632
• check cockpit-machines: test: Fix whitespace cockpit-machines#1317
• check cockpit-podman: test: Update pixel references to Fedora 39 tasks container cockpit-podman#1486
• check cockpit-ostree: test: Update pixel references to Fedora 39 tasks container cockpit-ostree#453
• check anaconda-webui: test: Update pixel references to Fedora 39 tasks container rhinstaller/anaconda-webui#52

[martinpitt]

Landing this now to reflect reality -- this doesn't preclude reverting the tag in an emergency, and we have a working cockpit PR to fix the remaining fallout.