use liquid-fixpoint < 8.10.7, which is deprecated (#7819)

Author: juhp

build-constraints.yaml

@@ -5015,7 +5015,7 @@ packages:
         - happy-meta
         - liquidhaskell == 0.9.12.2
         - liquidhaskell-boot
-        - liquid-fixpoint == 0.9.6.3.3
+        - liquid-fixpoint < 8.10.7
         - smtlib-backends
         - smtlib-backends-process
         - smtlib-backends-tests