Use the fixed AFP version of Finite_Map_Extras

[javierdiaz72]

While porting the Praos specification to the Þ-calculus (see #44), we came across an issue in which the GitHub automated build failed due to an abnormal termination of the Z3 prover. The source of this failure is a call to the smt proof method in the AFP's Finite_Map_Extras theory. Although the root cause seems to be related to GitHub Actions (as our code can be successfully built locally), a version of Finite_Map_Extras without such troublesome uses of smt would be the appropriate solution, given that @javierdiaz72 is actually the author of such AFP entry. We shall thus use the fixed AFP version of Finite_Map_Extras once it is available.