Add Stack 2.15.7

Stack 2.15.7 is required to use GHC 9.10.1, as detailed in the following
issues:

* commercialhaskell/stack#6540
* haskell/cabal#9856

Author: TravisCardwell

src/versions.json

@@ -65,6 +65,7 @@
     "2.4.1.0"
   ],
   "stack": [
+    "2.15.7",
     "2.15.5",
     "2.15.3",
     "2.15.1",