Revert "mkosi: temporarily disable panic_on_warn"

The BRTFS issue that caused a spurious WARN has been fixed and
backported to Noble, so we can enable panic_on_warm again.

This reverts commit 930d65c.

Author: bluca

mkosi/mkosi.conf

@@ -78,8 +78,7 @@ KernelCommandLine=
         oops=panic
         panic=-1
         softlockup_panic=1
-        # Disabled due to BTRFS issue, waiting for the fix to become available
-        panic_on_warn=0
+        panic_on_warn=1
         psi=1
         mitigations=off