Update CBMC dependency to 6.6.0 (#4050)

Kani now uses CBMC's latest release 6.6.0.

Resolves #4019 

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Co-authored-by: Michael Tautschnig <tautschn@amazon.com>

Author: qinheping

kani-dependencies

@@ -1,5 +1,5 @@
 CBMC_MAJOR="6"
-CBMC_MINOR="5"
-CBMC_VERSION="6.5.0"
+CBMC_MINOR="6"
+CBMC_VERSION="6.6.0"
 
 KISSAT_VERSION="4.0.1"